The chart-topping group Mediaeval Baebes yesterday helped the British Library to take formal possession of a manuscript whose importance as a medieval treasure is rivalled only by its place in the modern annals of tax minimisation.

The library held a ceremony to mark paying the last instalment of its share of the £15m cost of buying the Sherborne Missal for the nation. The early 15th century missal, brimming with paintings of birds, flowers and plants, is considered a masterpiece of English art and one of the finest surviving medieval books.

Three years ago its owner, the Duke of Northumberland, threatened to sell it abroad to pay his inheritance tax. The government waived his debt of £9.4m, leaving him due to be paid a further £5.6m for the value of the missal. Of this, the heritage lottery fund gave a £4.2m grant and the library raised £1.4m.

A family finance commentator wrote of the deal: "Ironically, inheritance tax tends to hit the less well off because they cannot afford to minimise their liability. The lesson for everyone, however, is that careful tax planning now could avert a hefty bill later".

The Mediaeval Baebes unveiled a public touch-screen page-turning version of part of the missal which joins digitised versions of the Lindisfarne Gospels and a notebook by Leonardo da Vinci.