Frank Roberts - now Sir Frank, and 89 - left London in autumn 1944 to take up his post as the minister in the British embassy opposite the Kremlin. He was at the Yalta conference between Churchill, Roosevelt and Stalin at the beginning of February 1945, and arrived in Moscow in mid-March.
There he struck up a close friendship with George Kennan, his opposite number from the United States, and the intellectual grandfather of US national security doctrine. Mr Kennan recently said Nato enlargement would be the greatest post-war error of US foreign policy, but Sir Frank disagrees. He believes Nato could provide stability in eastern Europe, just as it produced stability in Western Europe 50 years ago.
He describes the early days in Moscow with Mr Kennan: "We were doing the same jobs. I was number two to Archie Clark Kerr [British Ambassador] - he was number two to Averell Harriman. We got on extremely well. Our policies towards the Soviet Union were not very different."
Sir Frank met Stalin many times. "He was the most villainous man alive but a very astute one. He murdered even more people than Hitler."
Did Sir Frank know that at the time? "Of course ... But he wasn't a very dictatorial figure. Not like Hitler or Mussolini ... He was very softly spoken. So when Churchill and Roosevelt met him they were very pleasantly surprised."
Sir Frank wrote three masterly letters to Ernest Bevin, then Foreign Secretary, setting out the prospects for co-operation with Russia, Russian aims and attitudes. "George Kennan sent a big think-piece to Washington on the prospects of continuing what we still called the `Roosevelt policy' of getting on with the Russians. The Foreign Office then asked me to do a similar exercise from the British point of view - there being a slight difference between the British and American positions."
And what was that difference? "We had an empire. We were an imperial power. So we were more vulnerable to what Khrushchev later called "national liberation movements". The Cold War as such really came after the Berlin blockade. Before that we were just forecasting, as it were."
"The message really was we [Britain and the US] must be more realistic - if we are, we can get on with them."
Did policy towards Russia move in a different direction from that advocated by the diplomats in Moscow, I asked.
"It certainly did for Kennan in the US. The Roosevelt idea was, we must somehow get on. We, of course, had a very realistic foreign secretary in Ernie Bevin ... Of course, I was preaching to a slightly converted audience in the shape of Ernie Bevin - a great trade union leader ... Kennan's efforts were less successful in the US."
Sir Frank said that Truman "became a complete convert" to the "realistic" approach, but over time the Americans changed. "Containment became hostility. That was never his [Kennan's] intention."
Our conversation shifted to the present. Did Sir Frank think Western policy towards Russia was the right one?
"I think we have to always remember, whatever it is, the geographically- diminished Russia is still a potential great power and in some respects an actual one. The danger with Russia is that people either exaggerate its strengths or exaggerate its weaknesses..." But on Nato, he is unequivocal. "Russia can't be a member of Nato. it's too big. The Russian thing will have to be developed on its own - as one great power with a special relationship with Nato."
So is Nato right to enlarge?
"The Russians will never like a country like Poland going into Nato. I think it will result in greater stability in eastern Europe - but I couldn't expect to convince the Russians of that easily. I believe Nato has shown extraordinary resilience in modifying its position. It has to remain an effective military force but at the same time it has to take this great opportunity to provide stability in Eastern Europe, just as it produced stability in Western Europe 50 years ago."