It seems that Ramsay MacDonald had several affairs, one of which was with a Viennese socialite - Frau Kristina Forster.
This is according to a book review of "Former Panorama producer Andrew Williams's juicy historical novel" The Prime Minister's Affair entitled Oswald Mosley, Ramsay MacDonald and the pornographic blackmail letters (link behind a paywall)
In 1929 Ramsay MacDonald, embarking on his second term as prime minister, was threatened with blackmail by an ex-lover. A Viennese socialite called Frau Forster, newly ruined in the Crash, threatened to publish his “pornographic” letters to her if he didn’t stump up some hush money. That, anyway, was the story as related in old age by Sir Oswald Mosley, who claimed to have dealt with the problem at MacDonald’s behest by sending “the old girl” packing with threats of imprisonment (Mosley was then a Labour MP, and one of MacDonald’s ministers)...
The PM, his spurned lover and a cache of risqué letters mentions that:
The Prime Minister’s ex-lover was renting a flat in Horseferry Road – a poor district of London at the time – but only a few streets from Mosley’s grand house in Smith Square.
More about "Sex, blackmail, and Downing street" from Andrew William's site.