From "A Spy Among Friends", p. 280-2. Elliott was the man who finally caught Philby:

LATE ONE NIGHT a few weeks later, Elliott heard a soft thump as a letter, delivered by hand, dropped into the letter box at 13 Wil¬ton Street. Inside was a typewritten letter and an empty envelope addressed to Kim Philby at PO Box 509, Central Post Office, Moscow. The letter carried no postmark, date, or letterhead, but the handwriting on the envelope was unmistakable…

… I have often thought that there are a number of questions connected with the whole story that might interest you, and it might be helpful all round if we could get together as in old times and discuss matters of mutual interest. After careful thought, I have come to the conclusion that Helsinki, which you could reach without difficulty, would be a suitable rendezvous—or perhaps Berlin?

I am enclosing an unsealed, addressed envelope. In the event of your agreeing to my proposal, would you post it, enclosing some view of Tower Bridge? On receipt of your letter, I will write again, through the same channel, and make suggestions about the admin, side of the rendezvous…

Elliott was astonished. Philby wrote as if his betrayal were no more than a hiccup in their long friendship. It was as if Vermehren, Volkov, the pixies, and the countless others betrayed to their deaths had never existed. Was he trying to lure Elliott into a trap? Or was this an attempt to persuade him to turn double agent, to change his bearings in this “complicated world”? The sending of a blank postcard of Tower Bridge would convey a message that the KGB could interpret: was this intended to show that Philby was acting without KGB approval, and therefore a hint that he was prepared to be reeled back in, to “discuss matters of mutual interest”? Elliott’s initial reaction was one of outrage. “It was ridiculous to suppose that I would agree to meet him behind the backs of my boss and my wife.” The old Philby charm, laced with bravado, was there in abundance, with its allusion to their valued friendship and the hope that he had not damaged his friend’s career. Elliott decided that the letter must be “an incredibly clumsy piece of KGB disinformation, obviously designed to throw doubts on my loyalty.” The next morning he took the letter in to MI6 headquarters and showed it to Dick White, who was equally intrigued. The strange missive prompted “many hours of discussion as to what, if anything, should be done about it.” Elliott was in favor of setting up a meeting, “because first, I was certainly fitter than he was, and secondly because I could choose the rendezvous.” He was overruled.