Of all her various claims to distinction, Peggy Jay was always proudest of the 21 years she spent defending the heath and village in which she had grown up. Her name became synonymous with the preservation of Hampstead and the protection of its neighbouring open space against the depredations of the modern world. No area of London ever had a more zealous champion.

Although she was born in Manchester, where her father, Maxwell Garnett, was principal of the College of Technology, she was in every sense Hampstead-bred. Her parents moved to London from Manchester in l920 — on Garnett’s appointment as general secretary of the League of Nations Union — and, from the age of 7, Peggy Jay was to spend more than 70 years of her life as a Hampstead resident. But it was not just a question of an NW3 address: to many, at least in her latter years, she came to embody the whole political and social ethos that Hampstead is supposed to represent.

The product of a distinguished, progressive family (though her niece, Virginia Bottomley, was to end up as a Tory Cabinet Minister), Margaret Christian Garnett was educated at Malvern Girls’ College and St Paul’s Girls’ School before going up to Somerville College, Oxford, in 1931. She did not take a full degree course there, resting content with a two-year diploma in economics.

The reason for that was as much personal as academic. At the end of her first term her engagement was announced to a rising, young Fellow of All Souls who also happened to be the boy-next-door at home in Well Walk, Hampstead. Her marriage to Douglas Jay took place in 1933, when she was only 20. Although he was a mere six years older than her, he necessarily came to assume the role of something of a tutor and mentor towards her. (She was certainly very biddable, not even blenching when her husband-to-be announced even before they were married that he had always regarded monogamy as a sin.)

Peggy Jay was one of the last of a long line of emancipated, able women never to hold a paid job. Though by no means affluent, her family and her husband’s rated as “comfortable”, and it was simply not thought proper for the daughters, or wives, of middle-class families in those days to seek employment. A naturally energetic figure, Jay was to regret this all her life, and she certainly came to envy the relative economic freedom enjoyed by her daughters and grand-daughters.

Unlike many of her more placid contemporaries, she was never, however, likely to be content with sitting at home and doing nothing. Even by the time her first son was born in 1936, she was already active in Labour politics. In 1938, at 25, she became the youngest member to be elected to the old London County Council. She was to represent the borough of Hackney for 11 years, returning to County Hall, after a three-year gap, as one of the councillors for Battersea for a further 15 years in the 1950s and the 1960s.

In the very tightly run Labour group of that era she probably suffered slightly from her reputation for independence. Despite her long service, the only committee chairmanships she was offered were those of schools (much less powerful than education) and of arts and leisure. Neither counted as appointments of the first rank; it was also probably typical of the male- dominated atmosphere, at least when she first joined, that word should have gone out that “if lady members want to have children, they should please try and do so during the recess”.

Nevertheless, her defeat in the bleak electoral year for Labour of 1967 distressed greatly Jay — even if she had never wholly taken to the extended GLC, which replaced the LCC in 1964. The loss of her Battersea seat may well, though, have been a blessing in disguise. Certainly, it freed her for the kind of service on official committees at which she excelled.

She had already been appointed to the Royal Commission on Population in the days of the wartime coalition of government, and in her later years she was to serve on many similar bodies. She took particular satisfaction from her chairmanship of the Friern Psychiatric Hospital Management Committee, to which she was appointed by Richard Crossman, and — since her interests had started to run in this direction — also from heading a departmental committee inquiring into the training of staff who worked with mentally handicapped patients.

When this particular committee produced its findings, which raised a howl of rage from the nursing profession, Barbara Castle, the Secretary of State for Health and Social Security, who had established it, is reputed to have commented: “If I hadn’t expected a controversial report, I would not have appointed Peggy Jay.”

Although more than 50 years of her life had been spent in the Labour party, in 1983 Peggy Jay abandoned it and joined the SDP. By then she had been divorced from her former Cabinet Minister husband — who in l987 attained, at the age of 80, the life peerage that might equally well earlier on have been hers. It was greatly to her credit that, after the break-up of her 40-year marriage, she settled down to defending her native heath, to becoming the undisputed matriarch of a family that was growing through the generations, and eventually to writing, without rancour of bitterness, a highly evocative volume of memoirs entitled Loves & Labours (1990). She rejoined the Labour Party fold when Gordon Brown became Prime Minister.

She is survived by her two sons — the elder of whom was at the age of 41 British Ambassador in Washington — and by twin daughters, one of whom is married to the former Deputy Governor of the Bank of England, Rupert Pennant-Rea.

Peggy Jay, former LCC and GLC representative and chairman of the Heath and Old Hampstead Society, 1968-89, was born on January 28, 1913. She died on January 21, 2008, aged 94