Revealed: the secret legacy of John Paul Getty, patron of arts and oddities

The billionaire went on a spending spree late in life, donating £400,000 to 'The Oldie' magazine (he liked it) and a possible £12.5m to save Reynolds's portrait of Omai for the nation (more a case of hating America's Getty Museum). By James Morrison

Sir John Paul Getty II set up a secret trust to safeguard his £50m book collection for the British public as part of an extraordinary cultural spending spree in the last years of his life.

Seven weeks after the death of the American-born billionaire philanthropist, a clearer picture is now emerging of the full scope of his patronage. Among the substantial payments now being linked to his name are several made in the weeks before he died. These include a £400,000 lifeline for one of his favourite publications, the Oldie magazine, and the £12.5m pledged to the Tate by an anonymous benefactor to prevent Sir Joshua Reynolds's portrait of Omai being sent overseas.

The Independent on Sunday has also learnt that Sir Paul established the Wormsley Foundation in 1992, a sleeping trust dedicated to the preservation of his books. The existence of the secret foundation, which takes its name from the billionaire's sprawling country estate in Buckinghamshire, has been uncovered by The Art Newspaper.

Registered as a trust on 5 June 1992, its objectives are described on the Charity Commission's website: "The advancement of education, the preservation of historic and rare books and manuscripts, the encouragement of access and the promotion of study into such books and manuscripts."

Sir Paul, who died from a chest infection in April aged 70, is known to have been a passionate collector of antiquarian books. Five years ago, he paid £4.6m for an edition of Chaucer's Canterbury Tales printed by William Caxton in 1477. His last confirmed purchase was a First Folio of Shakespeare's plays, which he bought for £3.5m from Oriel College, Oxford, in March. He also owned such unique treasures as the Byland Bede, dated 1160, Boccaccio's collection of medieval morality tales, De casibus virorum illustrium, Anne Boleyn's psalter, and the Ottobeuren Gradual.

The manuscripts were housed in the cavernous library Sir Paul had built in his flint-clad castle at Wormsley, where he staged celebrity "Test" matches on a mini replica of Lords cricket ground. Conservative estimates put the archive's value at £50m.

The foundation's "area of operation" is stipulated as being England and Wales. However, its financial history, which began on 6 April 1996, reveals an initial capital payment of £100, a nominal annual income of £6-£8, and expenditure so far of "£0" a year - suggesting that, until now, it has intentionally been left dormant. Sources now say the foundation will be activated with money from his £1.6bn estate to make the collection available to the public.

Though little is known about the foundation's aims, its trustees are believed to be Victoria Holdsworth, Sir Paul's wife since 1994, his solicitor Vanni Treves and the London antiques dealer Christopher Gibbs. But a bemused Mr Gibbs said last night he had no knowledge of its existence. "It's absolute news to me, but it's very, very nice to know," he said. "I wasn't aware I was named as a trustee, but it's perfectly possible that I am."

Sir Paul allowed generous access to literary scholars in his lifetime, and the fact he was still buying books even when he knew he was dying - he had a long history of illness - is seen by some as proof that he had every intention of his collection surviving him. Anna Somers Cocks, editor of The Art Newspaper, said the foundation appeared to have been set up to ensure that the precious items were ultimately made public, adding: "People who put together fantastic collections don't on the whole want them to be dispersed when they die."

However significant the shadowy foundation, Sir Paul's patronage in the months leading to his death is thought to have extended far wider. Speculation is rife that he is the mystery donor who promised the Tate £12.5m to help it save the portrait of Omai, the Reynolds painting which was recently sold by the Castle Howard estate for £10.3m.

The picture, bought by an anonymous overseas collector, is currently in limbo in a south London lock-up, after the Government imposed a nine-month export bar to allow galleries time to match the auction price. Though the money is now available for them to do this, it is by no means certain that the original buyer will be willing to part with his prize once the temporary bar expires.

Though several suggestions have been put forward as to the identity of the mystery saviour, Sir Paul is now widely seen as the prime candidate. Of the other usual suspects, Sir Christopher Ondaatje, the millionaire financier and Labour Party donor, is seen as unlikely, as he is on the board of the National Portrait Gallery, which is trying to raise £1.8m to save another painting of Omai, by the Welsh artist William Parry. There are similar doubts about Lord Thomson of Fleet, as his acquisitions tend to benefit his favourite gallery in Ontario.

Sir Paul is also believed to have had a very personal motive for securing the portrait: to stop it falling into the hands of the Getty Museum, the vast Los Angeles archive established by his father, John Paul Getty I. In his will, the late US oil baron forbade his son from ever becoming a trustee of the museum, a factor some have used to explain Sir Paul's mammoth donations to rival UK institutions like the National Gallery, to which he gave a £50m endowment in 1985. Art world insiders insist that the most recent donation bears Sir Paul's stamp. One well-placed source said: "I am convinced it was him. It would be like a last great joke at the expense of the Getty Museum."

His long-time friend, Sir John Mortimer, added: "It would be quite typical of him, and I hope he did. It's a beautiful picture." The writer gave as an example of the billionaire's generosity his £1m donation in 1994 to the National Galleries of Scotland's appeal to save Canova's Three Graces.

Sir Paul also continued to support less fashionable causes, including The Oldie, to which he gave nearly £1m in his final year. Last week The Literary Review revealed he had also given generously to the magazine, but it kept the donation quiet until after his death.

For the past two decades, he even paid the running costs of the Vintage Wireless Museum in Dulwich, after being impressed by its service when he popped in to have a rare HMV Lumiere gramophone repaired. The museum's owner, Gerald Wells, 73, said of his benefactor: "He became a very good personal friend."

The wireless is not the only all-but-obsolete invention Sir Paul was willing to buoy up. Having bailed it out to the tune of £5m after the 2001 election, weeks before he died he was in talks to donate a further hefty sum to the Conservatives.