I first discovered Lawn Road Flats in Hampstead while working on a history of KGB foreign operations in the late 1980s with Oleg Gordievsky, a KGB officer who had worked for eleven years as an agent of the British Secret Intelligence Service (better known as SIS or MI6), supplying it with a remarkable range of KGB documents and other top-secret intelligence. Gordievsky greatly impressed the ‘Centre’ (KGB headquarters) as well as SIS. In 1985 he was appointed head (‘resident’) of the KGB's London station (‘residency’), one of the most important foreign postings in Soviet intelligence. Soon after his appointment, however, he was summoned back to Moscow for important consultations, only to discover that the KGB had discovered his work for SIS. Gordievsky then made one of the most extraordinary escapes of the Cold War, fleeing across the Finnish frontier in the boot of an SIS car en route to Britain.

I first met Gordievsky a year after his escape, at meetings in London and Cambridge during which we agreed to collaborate on the first of our three books. The first questions that I put to him included the two main unsolved mysteries about the ‘Magnificent Five’, five young Cambridge graduates recruited in the mid-1930s whom the KGB considered their ablest group of foreign agents. The identities of four of them – Kim Philby, Guy Burgess, Donald Maclean and Anthony Blunt – were already known. But controversy raged over who had been the Fifth Man.