The land was part of 725 acres purchased under pressure by the Air Ministry for a practice bombing range in 1938. Four years later Winston Churchill, as prime minister, promised that it would be offered back to the owners. Inevitably the postwar Labour administration had scant sympathy for the rights of the landowner, but it came as a shock when the Churchill government's return to power in 1951 produced no change in official policy.

Mary Anna was aided by her husband, Lieutenant-Commander Toby Marten, DSC, who resolutely knocked on official doors to no avail. She then brought the issue into the open by writing a letter to The Times. This mentioned "veiled nationalisation of land" and announced that a protest committee had been formed and that an inquiry was being demanded "as it appears that the rights of farmers and landowners and extravagance with public money may be involved".

While Mary Anna Marten was busy with what was to be their family of five girls and a son, her husband uncovered a trail of disingenuousness, dishonesty and bureaucratic incompetence until the revelations of the inquiry prompted rumblings on the Tory backbenches in the Commons, eventually leading to the resignation of the agriculture minister, Sir Thomas Dugdale, and the return of the property. Mary Anna Marten celebrated with a shopping trip to London.

Mary Anna Sibell Elizabeth Sturt was born on September 12 1929, the daughter of the 3rd and last Lord Alington of Crichel. The early death of her mother and father meant a lonely childhood. She joined the Brownies' pack at Buckingham Palace with Princess Margaret, and went to Miss Faunce's Parents' National Union School at Lancaster Gate, which moved after the outbreak of war to Wimborne St Giles, the home of her grandparents, the Earl and Countess of Shaftesbury. She then went to Cheltenham Ladies College.

Mary Anna met her husband on her first day at Somerville College, Oxford, where she read History – a cousin invited her to a lunch hosted by Commander Marten, who was not only dashing but as tall as she. Coming down from Oxford after a year, during which she had been a member of every political party, she married Marten in 1949 at Holy Trinity, Brompton, in the presence of the King and the Queen (her godmother) and Princess Margaret.

The couple lived in a farmhouse on the estate before moving into Crichel House in 1962 to embark on an ambitious programme of restoration. They pulled down an Edwardian wing, repaired the roof and improved the water supply. The house had been refurbished by James Wyatt in the 18th century, and the Martens were able to reveal an elegant upstairs salon which had been divided into three rooms.

Mary Anna Marten maintained an ambivalent relationship with the Conservative Party, which she considered neglectful of rural interests. She was a patron of many local charities, and a member of the appeal committee for the spire of Salisbury Cathedral.

As her children grew up she became a trustee of the British Museum, taking a particular interest in the Romano-British department and its staff, and chairing the buildings and development committee for the Great Court, which was completed for the Millennium.

She was an equally enthusiastic trustee of the Royal Collections, going on one of its courses for curators, and also visited the Hermitage in St Petersburg, setting up the Ancient Persia Fund in memory of its orientalist Vladimir Lukonin.

Mary Anna Marten, who was appointed Deputy Lieutenant of Dorset and OBE in 1979, died on January 18. Her daughter Victoria McDonaugh attended the funeral as High Sheriff in the dress and hat which her mother had worn when she had held the office.