SIMON SAINSBURY was very much the self-effacing member of the influential fourth generation of the Sainsbury grocery dynasty. His brothers are John, Lord Sainsbury of Candover, and Sir Timothy, the former Conservative Trade Minister, and his cousin is David, Lord Sainsbury of Turville, the Science Minister in the present Government, all of them powerful board members of the group that presided over a surge in the chain’s prosperity in the 1970s and 1980s, and which has spread its authority widely with philanthropy in education, science, the arts and heritage.

But reserved and retiring from the public gaze as he always was — he shunned honours and refused to have an entry in Who’s Who — Simon Sainsbury had a profound individual impact on museums and the heritage in particular.

As far as possible he did his work surreptitiously, through the Monument Trust which he created in 1965. The latest project he was concerned with was at Pallant House, the Queen Anne house at Chichester near his Sussex home, where an £8.6 million wing for modern art was opened in July to house the donation of the collection amassed by the architect Sir Colin St John Wilson.

Simon David Davan Sainsbury was the middle son of Alan Salisbury (later to become a Labour peer). He went to Eton, as did his younger brother, Tim, where he was a model Etonian, excelling at sport and music and becoming head of his house and president of “Pop”, the Eton Society.

He served his National Service in the Life Guards — where he was sports officer — after which he went up to Cambridge to read history at Trinity College.

After training in chartered accountancy he joined the family company’s finance department, and three years later was a director with responsibility for finance and personnel. In 1967, on the retirement of their uncle Robert, Simon became deputy chairman to the chairmanship of his brother, John, and by the time John Sainsbury retired in 1992 Sainsbury’s had a turnover of £9.2 billion. He and his brothers, Simon said, had been fired by the ambition to run the business “better than it had ever been run before”.

It had been Simon’s responsibility to steer through the transition of Sainsbury’s from a private company to being a publicly quoted plc through which a million shares were made available to staff in what was dubbed in the press “the sale of the century”. The applications were closed within one minute.

With the flotation — it was the largest on the London Stock Exchange — the group suddenly had a new, public image which brought its leading members under an unaccustomed spotlight, to Simon’s particular alarm. Before the flotation the family’s philanthropy had been generous and public, but afterwards it became clear that their own giving, as opposed to company sponsorship, would attract interest that distracted from the projects themselves, so their own giving was to be done through individual trusts — John and his wife, the ballerina Anya Linden, created the Linbury Foundation, which is especially concerned with the performing arts.

Simon named his the Monument Trust after the house he was renting at the time, and in its 40 years Sainsbury gave more than £100 million. He also supported other charities outside the arts and was a long-time supporter of research into HIV/Aids.

Sainsbury’s particular interest was 18th and 19th-century architecture, and Impressionist painting, of which he had an impressive collection at his houses in Sussex and Chelsea. As with the other family trusts, projects under the Monument name were usually funded exclusively.

In its early days the trust was a key supporter of the Georgian Society, and led the way to addressing the crisis among great houses by helping to save many from threatened demolition, assisting their conversion to new purposes — typically to comprise flats, conference centres and even museums but retaining their architecture — for which their owners did not have the resources.

The trust played an important role, over 15 years, in the regeneration of the Meadow Well housing estate in North Tyneside after the 1991 riots.

On the scores of a projects he gave to, only the Monument name was credited, but anonymity was impossible when the three brothers came together to fund the creation of the National Gallery’s new wing, which opened in 1991. It was Simon Sainsbury who led the project management, chairing committee meetings sometimes three times a week over four years to steer the development through the complexities of a controversial but fundamentally important scheme. It was thought to have cost about £35 million, but so much were the Sainsburys in charge of the project that they received all bills and never revealed a final cost. Simon took a thoughtful interest in all involved (including the stonemasons and car park attendants, for whom he organised a party).

One of the museums he helped to re-establish quietly was the Wallace Collection at Marylebone, of which he was chairman of trustees for 20 years until 1997. He was a trustee of the National Gallery from 1991 until 1998.

“He was very good company and extremely knowledgeable about British art, and was that rare thing — an empathetic donor on all the many projects he was concerned with,” said Sir Nicholas Goodison, the former chairman of the Art Fund, whom he consulted on a number of projects. “He was always prepared to ask the question ‘why’ at the right point, and when he was satisfied with the answer would be extremely generous.” Friends admired his formidable self-discipline and dry sense of humour.

Sainsbury was closely concerned with the restoration of Hawksmoor’s most famous building, Christ Church Spitalfields, and through the Monument Trust supported the British Museum, the Tate, the Victoria and Albert Museum, and the Fitzwilliam Museum, Cambridge. Without his support the original Addenbrooke’s Hospital building in Cambridge would not have been saved from dereliction, and it has now been restored to be the Cambridge Judge Business School.

His partner for 40 years was Stewart Grimshaw, a restaurateur and, secondly, bookseller, with whom he registered a civil partnership earlier this year. Latterly Sainsbury suffered with Parkinson’s disease, the cause of the fall which led to his death.

Simon Sainsbury, businessman and philanthropist, was born on March 1, 1930. He died on September 27, 2006, aged 76.