Her social and political concerns took her to many places where there was deprivation or conflict, be it the East End of London during the Second World War, or the West Bank at the time of the Palestinian intifada. In her sixties and early seventies, when most ladies in possession of a chocolate- box cottage in an English country village would have chosen to stay put there, she was still relentlessly travelling, sleeping out under the stars with the Polisario women of the western Sahara, or sitting in a mud hut in a remote part of south-east Angola, while Jonas Savimbi's rebel commanders tried to persuade her that they were really Liberal Democrats at heart.
Penelope was the third of the five children of the fine but extremely difficult Oxford bookseller and publisher Sir Basil Blackwell. As a girl, Penny would have no major stake in the family business, though having an elder brother at the Dragon School in Oxford meant that she was able go there. Following a spell at St Leonard's girls' boarding school in St Andrews she went up to Somerville College, Oxford, to read Greats; academically brilliant and beautiful, she seemed doubly blessed.
The Second World War broke out, and she joined the ATS. Then in 1940, at the age of 20, she married Robert Jessel whom she had met at Oxford. There was bitter opposition from her father, partly because the Jessels were of Manchester Jewish immigrant stock - though in fact Robert's father, a doctor, had married out of and abandoned his faith.
Bobbie Jessel went on to become Defence Correspondent of the Times. But the couple's happiness was relatively short-lived. He died of leukaemia in 1954, leaving Penelope a young widow with two young sons to care for - Stephen and David, both later journalists. She responded to the challenge by acquiring qualifications that enabled her to become an adult education lecturer in social administration and social work, notably at Plater College in Oxford.
Unlike many professionals in those fields, she did not become an ardent Labour supporter. Instead, inspired by Jo Grimond's vision of a Liberal revival, she joined the Liberal Party and carried its banner in half a dozen parliamentary elections in the 1960s and early 1970s, in various hopeless seats.
She fought the May 1965 by-election in Birmingham Hall Green, operating out of a poky caravan. The contest was uninspiring and low-key, but Penelope added colour to it, the Times reported, by looking "like a jolly and elder sister of Pussy Galore". The young Peter Preston, writing in the Guardian, declared that she was "one of the most adroit and charming canvassers extant". It was all to no avail; the Conservative cruised comfortably to victory, though Penny Jessel did avoid the classic third party squeeze.
Denied the opportunity of serving in the House of Commons, she devoted herself to working both inside and outside the Liberal Party on women's issues and international affairs. From 1985 to 1988 she was the party's International Officer (unpaid), having already become a familiar figure at Liberal International Congresses. She was hurt by the way she was eased out of that position, to make way for a younger (paid) person. But she had the consolation of being made a Dame in 1987, to mark the centenary of the Women's Liberal Federation. Had the Liberals had a more generous allocation of peerages, she would have had a strong claim to one of those.
Jessel listed among her recreations looking at churches and gardens. The reality behind those innocent-sounding occupations was years of fierce campaigning on conservation matters, especially in Oxfordshire; she was a tenacious fighter and a ferocious letter-writer when the subject was dear to her heart.
She was also a chain-smoker of formidable proportions. Eating in restaurants with her became a battle of wits to see if one could eat slowly enough to prevent her lighting up between all the courses. She usually won.
She bore the cancer that killed her with immense dignity and carried on her voluntary work, mainly for the Liberal think-tank the John Stuart Mill Institute, right up until her death.
Penelope Blackwell, political activist and lecturer: born Oxford 2 January 1920; President, Women's Liberal Federation 1970-72; International Officer, Liberal Party 1985-88; DBE 1987; married 1940 Robert Jessel (died 1954; two sons); died Cassington, Oxfordshire 2 December 1996.