Of course he doesn't have the long hair any more, or the flares, or the saucepan of lentils. But the perennial gloom of Neil still clings to Nigel Planer - 20 years after The Young Ones, people still shout 'Heav-ee!' when they see him, or 'Cheer up, mate, it may never happen.' He has that sort of hangdog expression and over-anxious manner. And yet Nigel Planer - or rather, his alter ego Nicholas Craig - wrote one of my all-time top five funniest books, I, An Actor, which still makes me shout with laughter. (Happily, Methuen is planning to republish it - anyone who ever has to deal with actors should keep a copy handy.) And now he is emerging as a seriously good novelist. His first novel, The Right Man, published in 1998, was promising, but his second one, Faking It, which comes out this month, is excellent, a sort of cross between Nick Hornby and Kingsley Amis, if you can imagine such a thing.

But, maddeningly, he is an actor first and currently very excited about a new play he is in, called Feelgood, by Alistair Beaton, about the backstage spin-doctoring at a Labour Party conference. I saw it at Hampstead Theatre and found it amusing - but that's never enough for actors: you have to say it's the most side-splitting comedy you've ever seen in your life. Visiting Planer at home the morning after the first night, I found him frantically leafing through the papers in search of reviews - luckily, there was a good one in The Guardian with a 'nice mention' for him. He said it was vital to get good reviews because then the play would get a West End run (it is currently on tour) and he'd get some proper dosh, instead of the £200 a week he is paid out of town. He stays awake at night worrying about money. He has a new wife and a new baby to support.

They are currently living in hideous chaos in a tiny rented house in Twickenham while their slightly larger new house opposite is being done up. They can look out of the window and see the builders shovelling money into the moneypit - they have been at work on the house for three years . It is a precarious business, living in this sort of borderland of celebrity. On the one hand, he is 'famous' enough for Hello! to pay for exclusive coverage of his wedding - on the other hand, he is obviously not rich. Compared to his fellow Young Ones, Rik and Ade, he is definitely the poor relation.

He shot to fame with The Young Ones in his mid-twenties and spent most of his thirties coming down. 'I was working so hard in the 80s, I kind of tried to do everything and I reached a burnout, I think, because I was with a group - Rik and Ade, who are so brilliant, and Alexei Sayle and French and Saunders - and I kind of made a mistake, I suppose, in trying to be like them.' He kept writing television scripts which were meant to be starring vehicles for himself, but finally 'I had to realise I'm not that kind of person - I'm not a personality.'

I see what he means - he is not 'larger than life', thank God, he is not starry. But he is an actor with an actor's fear of how he'll be perceived by the public. Occasionally he'll be talking away and suddenly stop and say, 'Oh God, you're a journalist!' and start fretting about how some anodyne remark might be misconstrued. He seems to imagine an audience of Guardian - Observer wimmin scanning his speech for any hint of sexism or politically incorrect views. I asked at one point whether he'd ever been to call girls (because both his novels contain sympathetic characters who are call girls) and he almost yelped with fear and said no, no, he couldn't possibly condone the sex industry or the exploitation of women. Which of course wasn't quite what I asked.

But that is what makes his novel, Faking It, so intriguing - the constant flip-flop between two views of masculinity, personified by the two main characters. Oliver the osteopath is a goody-goody New Man who attends men's groups and worries about caffeine; Barry is an unreconstructed Neanderthal journalist who is pissed all the time. Planer admits that both characters are based on him, or rather, both are exaggerated versions of two sides of his personality. Superficially he is much more like Oliver - earnest, health-conscious, high-minded - but he says he had a Barry period when he was living on a houseboat in Chelsea and drinking too much between marriages (and perhaps also observing the call girls at Chelsea Harbour). I don't want to give the plot away, but whereas Oliver appears to have all the virtues and Barry all the vices, by the end the position is pretty well reversed.

For all its playfulness and comedy, Faking It deals with serious moral issues - about how to be a good man and not a bastard; how to have a serious, caring, mutually respectful relationship despite having a bit of a penchant for top-shelf magazines. I suppose the concerns are very specific to Planer's generation - the now-fortysomethings who came to manhood just in time to be told that a woman needs a man like a fish needs a bicycle. Nick Hornby and Tony Parsons cover similar ground but not, I think, as deeply and honestly as Planer. They manage to fudge the sex issue by largely ignoring it, whereas he tackles the essential problem that the penis is a wayward beast which doesn't necessarily sign up for all the tenets of feminism. Can you say you're in love with your partner if you still have fantasies about spanking Miss July?

Planer told me that he once read a women's magazine (he often reads women's magazines) in which two agony aunts - one a Relate counsellor and the other a page three girl - answered a question from a male reader, viz: 'I want to look at porn with my girlfriend because it turns me on, but she thinks it's disgusting.' The page three girl thought it was OK as long as he explained to his girlfriend that it didn't mean he didn't fancy her. But the counsellor said: You must touch a woman gently and buy her flowers and absolutely no porn.

'Which I thought,' Planer concludes, 'was completely unhelpful and probably going to ruin their relationship. In a perfect world, he should have been able to do a deal with his girlfriend and say, I'll give you the roses and the warmth and the intimacy thing, if once a month or so we can look at porn together. Then they would have had a shame-free relationship. Barry in the novel's got a lovely line where he says: "I just want some depersonalised voyeuristic sex with the woman I love every now and then. Is that too much to ask?" But he does say with "the woman I love", not the woman I want to exploit.'

Planer's own love life has been complicated. His first partner was much older than him, his two wives much younger, he has never gone out with anyone his own age. In fact, he says he has never 'gone out' with anyone - all his relationships have either been casual or deeply serious. 'Maybe I'm a dinosaur. I mean, it's more fun to play the field, quite honestly, and if you're going to commit to somebody, then why not bloody get on with it - why do things by halves?'

He fell into his first relationship, with Roberta Green, because he rented a room in her house, so he was her tenant before he was her lover. When they met, she was a speech therapist (like his mother) but he helped to support her while she re-trained as an analyst and she is now quite well-known. He was in his early twenties, she was in her late thirties, and had two teenage children. It made him grow up very fast - he feels he jumped straight from adolescence to midlife crisis. 'Because I was thrown in at the deep end - the children were 12 and 10, and I stayed for eight years, so I went right through their teenage years when I was in my mid-twenties, which is a helluva battle - too young to be a father to them and too old to be a friend. I think, probably, if you were writing the plot of my life, that was the inciting incident.'

Why do young men fall for older women? 'I can't answer for anybody else. I just know how boring all the other girls seemed in my mid-twenties, and how scary as well. They want all sorts of things and they've got no wisdom. It was much more interesting to have someone you could actually talk to.' But surely there must be some deeper explanation? Oliver the osteopath in Faking It has a relationship with a much older woman and says he sees it as 'a way of getting out of having to have children'. But Planer says firmly that that's Oliver speaking, not him.

He is still good friends with Green, her children and, now, her seven grandchildren. But when that relationship broke up, he married a much, much younger woman, Anna Leigh, a model whom he met at a disco in Lanzarote - she was 22, he 35. Again, there was no courtship because she got pregnant almost immediately - 'It was a mad romantic rush and then it was: Right, let's decorate the nursery.' He loved having a son, Stanley, but the marriage was 'an absolute catastrophe' and ended in 1995.

This is when he had his Barry period, living on a houseboat in Chelsea, drinking too much and sowing his wild oats. He felt he'd missed out in his twenties, when he was being a stepfather, and anyway, 'It's easier to be a lad when you're in your forties,' he believes, 'because you can sort of take it or leave it. You're not so puppy-like and it's more of a laugh.' So was he very promiscuous? He goes all panicky at the question. 'I don't know that I should answer that! I have done that, but not as a habit.'

Anyway, soon he met Frankie Park, an actress who was also a single parent with a son the same age as Stanley. They married in 1999 and had Harvey, who is now 18 months. It was Frankie who said 'Go for it' when Hello! offered 'serious money' for exclusive coverage of their wedding and new baby. 'She's much more extrovert than me anyway, much more confident and outgoing.' She is also gorgeous looking and, he says, a 'bloody funny' actress, but the roles are few and far between. Like him, she has turned to novel-writing, and is now working on her second.

Before Planer wrote novels, he wrote sort-of spoof books, on acting, on therapy, on divorce. Ironically, the latter, Let's Get Divorced!, came out just when his first marriage was breaking up and it suddenly wasn't funny. 'I was having to do interviews, with everybody asking so what about your divorce? It was a bit of a turning point to realise that discretion is probably required, whereas before that I thought, Anything's a joke, really. So then I decided to write fiction instead because - I think it was Graham Greene who said the difference between a journalist and a novelist is that a novelist can write the truth and the journalist has to write lies.'

His first novel, The Right Man, is about a man whose marriage has broken up and who is fighting to maintain contact with his child - obviously to some extent autobiographical. Planer didn't want to be a Saturdays-only father, he wanted to be a full-time dad, and he assumed that having shared the childcare during the marriage, he would continue to share it after the divorce. But he had a rude awakening: English courts still almost automatically award custody to the mother. In the end, he became 'a part-time full-time dad' and had Stanley to live with him for a third of the year. But now, much to his sorrow, his ex-wife has remarried and moved abroad and he only sees Stanley in the school holidays. 'I don't want to go into it too much - anything sets me off - he's only just gone, only a year ago.'

In his post-divorce anguish, Planer got deeply involved with men's groups, including Families Need Fathers, which campaigns for fathers' rights. He still attends their meetings and has written the introduction for their new handbook. But his novel contains a wickedly funny send-up of a men's group - The Men Who Run Away from Wolves - and its hideously sanctimonious leader. It is hard to see how anyone could ever attend a men's group after reading it. So where does Planer stand? 'I'm still on the fence. I think there is a sort of awakening with men, but it's very difficult to talk about it without making it sound chippy or rightwing or anti-women. But there are a lot of issues - it is true that there is no widower's pension; it is true that if you're not married the woman can change the name of your children and take them away, and all of those issues are discriminatory and sexist against men. And men don't believe they have a right to have feelings in the home or to be sad or to cry - in the home what they have to face is isolation and alienation.'

He is a veteran of groups and therapies, analysis and alternative medicine. He wrote a good book called Therapy and How to Avoid It , which, though a spoof, is actually quite informative. He warns particularly to beware of Kleinians who believe that treatment only ends when the analyst dies. He had a go at one-to-one analysis but found it unsatisfactory - 'because there's a danger, as an actor, that you can beguile the therapist'. He preferred group therapy and went to it for years, though he has recently stopped.

But why does he need all this therapy - why is he so screwed up? 'Well, thank you for asking that! I'll need another six years in therapy to decide that one! Maybe I'm not really screwed up. I kind of use it because it's all I've got, to make comedy and writing. I don't think I'm that screwed up. I am very highly strung.'

OK - why is he so highly strung? It sounds as though he had a perfectly normal childhood - he is the middle of three brothers, and very close to the others. His parents were married for 56 years till his mother's death last year; she was a speech therapist, his father ran a company making medical equipment. They lived in Sheen and there was enough money to send all three sons to fee-paying schools - Westminster, in his case. The only remotely odd thing was that his father was Austrian and 'very European, just better , really, more olive oil in the blood or something. So that sort of gung-ho British maleness, that anal British thing about, You must be a poof if you're not behaving like a complete dickhead - I just don't get it.'

But it sounds as though he was pretty weird as a young man. His weight fluctuated alarmingly - he was very skinny as a small child, then so fat at school that he was called Piggy, then very skinny again. 'When I was 21, I was 8st - which at 6ft 3in is very thin - I had to go into hospital. And then by my mid-thirties, when we had a baby - I suppose it's couvade - I was nearly 16st.' He had a druggy period at Sussex University, then a period of being an intense fitness freak. When he was doing The Young Ones , he was so obsessed with his diet he had to take all his own bread to Australia. 'For four years, I didn't smoke or have caffeine, or foods with any "e" markings. I couldn't have any chemicals at all. I was just detoxing. But for four years! I went bonkers, It cleared up my eczema and allergies, but I was completely neurotic and irritable. Fussy, you know, and sort of unpleasant and stroppy. And then when I decided against that and thought, No , bugger it, I'll go mad, I'll have lots of meat and cigarettes and wine whenever I feel like it - of course then you put on weight and get a bad back. But it's much healthier, I think, if you just say what you think and do what you like.'

Hmmm - but he still seems to have some hang ups about food. He spent ages dithering over the menu at lunch, debating whether or not to have 'the full meaty thing', then spent at least an hour pushing a slice of Parma ham round his plate. Until recently, he confides, he suffered from a bad back, bad sinuses and irritable bowel syndrome and used to go to different specialists, but finally cured the whole lot by wearing a plastic denture for a year to alter his bite. 'It sounds crazy, but I'd say I'm half an inch taller than I used to be, and the constant pain and symptoms have gone - isn't that extraordinary? You say it to people and they think you're a nut, but I am healthier by biting differently.'

So now, in theory, he is 100 per cent fit. He has stopped going to group therapy, stopped worrying about his bowels and sinuses and bad back. Now all he has to worry about is money. He makes his living from many different sources - writing books, theatre acting (he did a year in Chicago in the West End); television acting (he plays the father in The Grimleys); lots of voiceovers and audiobooks (he reads all the Terry Pratchet novels). He also has a production company, Elephant, which produces mainly children's cartoons (some of them written by his brothers) and says that, after seven years, it is finally in profit. And now he has written Faking It, which certainly deserves to be a bestseller.

He already has plans for his third novel, which he wants to call The Long Night of the Penis. This time, he says, it will have a main woman character who is not dead. But it all depends on whether he can afford to take six months off to get properly stuck into the novel and whether Feelgood will go into the West End. Selfishly, I want him to fail as an actor so he'll be forced to become a novelist, but I fear he would prefer the reverse. He says he's been acting for 30 years and 'It kind of gets better the older you get. The competition's less and it doesn't matter what you look like. And what's so brilliant about it is from a very early age you work alongside people of all generations and all different classes, from all over the country.' And all different personalities? 'Well no, in that sense, not so good, because they are all actors!'

• Feelgood is at the Hampstead Theatre, 98 Avenue Road, Swiss Cottage Centre, London NW3 (020 7722 9301) until 10 March.