She does her supermarket shopping online, has a soft spot for Norma Major, and thinks her husband would never have married her if she couldn't cook. On the eve of her 50th birthday, welcome to the private world of Cherie Blair.

The Prime Minister's wife yesterday offered a rare glimpse of life behind the famous Downing street doors, as she embarked on a whirlwind round of publicity for her forthcoming book on Prime Ministerial spouses - entitled, appropriately enough for someone never out of the public eye, The Goldfish Bowl.

It seems she has learned from past encounters with the press: the cheerful chaos amid which she impetuously allowed Marie Claire to photograph her last year, with baby Leo's toys scattered across the floor and knickers spilling out of her bedroom drawers, was notably absent from this weekend's photoshoot for The Daily Telegraph, which is serialising the book. Dressed impeccably by British designers, Cherie poses in a living room ruthlessly tidied of all telltale personal clutter.

But the woman who describes her role - like that of the other Downing Street spouses she interviewed for the book - as being 'someone who is totally there' for her husband still could not resist a few moments of candour, admitting that she had slipped up along the way - not least during the media storm two years ago prompted by the revelation that she had bought two flats in Bristol with the help of Peter Foster, convicted fraudster and boyfriend of her assistant Carole Caplin.

'Not one of my happiest episodes ... Like everything else in life, some things you wish you had done better but of course you don't know that at the time. My greatest regret was that it distracted Tony from what he should have been doing,' she said. 'I was there to help him, not to cause him problems.'

Judging by yesterday's interview, however, she is determined not to do so again. Denying rumours that she had disagreed with her husband over the Iraq war, Mrs Blair was nonetheless careful not to be drawn on policy, insisting the last thing she had successfully persuaded her husband to do was to give up smoking 25 years ago: 'As it happens, I do agree with him (about Iraq). But that is not really the point, is it? The point is that it's his job to make those decisions, not mine.'

Her book, written with Cate Haste, wife of Lord Bragg, focuses on 10 former Downing Street spouses, including Denis Thatcher and Norma Major. Margaret Thatcher, she says, she found 'charming' but she clearly warmed to Norma, like her the daughter of a working mother who supported the family:

'She was never a drab woman. I had a view of Norma from the papers but when I met her, I thought, she doesn't appear to be this mouse-like creature. She is rather calm and confident.'

As for the current first couple, there were tantalising glimpses of ordinary life - installing showers for 'hosing down' her teenage sons after sports, and how she stopped shopping on eBay after newspapers found out she was using it to buy shoes - and of her marriage. Her purpose, she says, is 'about being an anchor, someone who is here totally for him and not having an agenda that is anything other than supporting him. And in politics, of course, that is not always the case with anyone else.'

But she was careful to duck questions about her husband's greatest achievement - 'I'm not getting into that. We are not by any means at the end of the story' - or about their retirement.

Yesterday's interview marks the first salvo in an autumn publicity offensive, with Cherie lined up for the Richard and Judy show next week and then a lecture tour of the US. But it seems there is a limit to how much of her privacy she will sacrifice, even in the name of book sales: last night's party at Chequers, the Prime Minister's country residence, to celebrate both her birthday and her book was kept strictly under wraps, with a guestlist of trusted old friends - and very few politicians.