While at Somerville College, Oxford, she began her first relationship with Tony Bray, an army cadet she met through the Oxford University Conservative Association.
At tea in one another's rooms she proved herself a "good housekeeper" with her cooking of crumpets and in March 1945 he presented her with a spray of carnations as the pair attended a ball at the Randolph Hotel.
Asked about it 60 years later, Mr Bray broke down in tears when reminded of the blue dress worn by his date, saying: "It was a very special evening."
But the relationship fizzled out as Mr Bray's military training took him away from Oxford although when he returned to the area in 1948 she heard from him again and had several more dates.
By 1949 she was working as a research chemist at BX Plastics at Manningtree in Essex, where she met Mr Cullen and had been selected as the Conservative candidate for Dartford, where she was first introduced to her future husband.
Despite going on a series of dates with Mr Cullen, Lady Thatcher seemed keen for him to meet her sister, writing to her: "Went to the flicks yesterday with my farmer friend and got him all primed up to meet you sometime.
"I showed him the snapshot of you and I together - and he said he could scarcely tell the difference so I should think we could easily substitute me for you. When can you come down for a weekend?"
On April 8, 1949, only a few weeks after her relationship with Willie had begun, she had Muriel to stay in Colchester and introduced her to him.
But Mr Cullen persevered in his attempts to woo the future prime minister, giving her
"frightfully expensive" Crepe de Chine scent and a "very nice" handbag with her initials on it.
She wrote: "I'll have to hang on with William for a while longer now!"
But in the same letter to her sister she said she was going to the North Kent Rotary Ball "with a chap called Denis Thatcher who is managing director of the Atlas paint works in Erith . . . He's all right - but is most unpopular with his men. He's far too belligerent in dealing with them and they naturally don't like it."
By January 1950, the process of shifting Mr Cullen's affections to her older sister was well under way, as disclosed in a letter to Muriel.
"I have written to William in the vein I told you. He wrote a letter to me - much warmer in tone than his others and the two must have crossed in the post ... We are meeting in London on Saturday afternoon to talk over the various aspects of 'we three' and it will then be broken off between he and I sic, for good and all."
The meeting did not take place, but the pair discussed the situation over the telephone and "I told him from henceforth that I would 'in law' only be taking a sisterly interest in future."
"He seemed quite satisfied and is quite pleased with 'future prospects'."
On Valentine's Day 1950 Mr Cullen and Muriel Roberts announced their engagement and were married in April that year with Lady Thatcher, who failed in her attempt to win the safe Labour seat of Dartford at the election in February, as their only bridesmaid.
- PAA
