By Gordon Corera
BBC Security Correspondent
British spy Kim Philby handed over secrets to the Soviets
The world of espionage lies at the heart of the mythology of the Cold War.
Along with nuclear weapons, spies were the emblems of the conflict.
But while the tales of adventure, betrayal and mole hunts have proved a source of rich inspiration for thriller writers, did they actually make a difference to the outcome?
Did intelligence make the Cold War hotter or colder?
It is difficult to know the answer.
"There were secrets that were important to keep secret and there was intelligence which it would be very helpful to have known," argues former British Foreign Secretary David Owen.
"But my own instinct is that we didn't really - with a few exceptions and a few important exceptions - really know exactly what was going on."
One reason it is hard to make a judgement is that much of the intelligence collected was military or tactical in nature, and would only have proven useful if the Cold War had gone hot.
Much effort was expended in stealing secrets like the Soviet order of battle or the design of new Soviet tanks which would have been invaluable in case of war.
This type of intelligence was collected by electronic means and satellite reconnaissance, as well as by human spies. It was used to work out how to best equip and prepare the military.
Sir David Omand, the former UK Intelligence and Security Coordinator, says: "Intelligence during the Cold War had a very big impact on the shape and size of the British defence programme, on the kinds of equipment we bought and very specifically the actual capabilities that were built into that equipment to be able to encounter whatever intelligence showed was the capability of Warsaw Pact forces."
During times of "hot war", intelligence plays an important but ultimately secondary role in supporting military operations.
But, during periods of tension short of full-scale military action like the Cold War, intelligence takes on a more central position.
In the absence of traditional warfare, intelligence becomes itself the primary battleground as each side tries to understand the enemy's capabilities and intentions, as it seeks to undermine their position using covert action, psychological operations and forms of subversion.
Britain's Secret Intelligence Service (SIS or MI6) had a troubled beginning to the Cold War, not least because it was penetrated by its Soviet counterpart, with men like Kim Philby and George Blake handing over secrets.
But slowly it became more professional, recruiting and running agents who could provide information on the activities of the Soviet bloc.
Some former diplomats query the record of intelligence in providing insight into political trends.
Rodric Braithwaite, a former ambassador to Russia at the end of the Cold War and later Chair of the Joint Intelligence Committee, is something of an intelligence sceptic.
A Soviet spy's insights changed Thatcher and Reagan's approach
"I was always rather encouraged by the Joint Intelligence Committee, who used to send us drafts of their assessments on Soviet affairs with the secret bits cut out because they didn't want to have them sloshing about in Moscow.
"With the secret bits out, the conclusions they were coming to were exactly the same ones that we were coming to in Moscow because the information that mattered was available at both ends and it was mostly either conversations with people, which were not particularly secret, or what was in the newspapers."
But Sir Gerry Warner, a former deputy chief of MI6, believes intelligence helped ensure politicians had a realistic understanding of what the Soviet Union was up to.
"It is always a temptation if somebody is saying 'I am a friend of yours and I don't mean any harm' to accept that.
"But if you are being told all the time by a microphone in your ear that it is totally untrue and that he's holding a knife behind his back, he's about to kick you where it hurts, the temptation is less to trust him."
Running agents behind the Iron Curtain involved risk - risk to the life of an agent but also politically in terms of raising the temperature.
"The main concern was always balancing the value of possible intelligence against the risk," explains Sir Gerry Warner.
"If an espionage operation was uncovered it was always an important public event - the media got into it, the other side would play it up - and therefore there was a political risk clearly."
Spy rows flared periodically. In the early 1970s, the UK expelled more than 100 Soviet diplomats from its embassy in London.
So did these kind of operations and activities fuel distrust and paranoia?
The identity of most agents remains secret but a few have become public and one or two of those can be claimed to have made a real impact.
One was Oleg Penkovsky, a colonel in Soviet military intelligence.
His information - passed to MI6 and the CIA in the early 1960s - helped President Kennedy manage the Cuban missile crisis successfully by identifying the extent of Soviet missile capability and how far the Soviet leader Nikita Kruschev was likely to push events.
The most useful strategic intelligence comes from penetrating the leadership of your enemy so that you understand not just their military capability but their intentions.
That was something MI6 only managed late in the Cold War largely thanks to KGB Colonel Oleg Gordievsky, who spent a decade towards the end of the Cold War supplying intelligence to MI6 which revealed how paranoid the Soviet leadership was of a first nuclear strike by Nato.
"The British service could not believe it but because I proved it very well they eventually believed it," he said.
"Knowing your enemy is very important indeed," argues Baroness Daphne Park, a former MI6 controller.
"It was very important that we should know that they were as paranoid as that. I don't see how we would have known it any other way."
Col Gordievsky's insights had a profound effect on both Margaret Thatcher and Ronald Reagan in rethinking how they approached the Soviet Union, which in turn helped them manage the end of the Cold War.
"What nobody wanted was to be surprised," Sir John Scarlett, the chief of MI6, told me in his office.
"And that intelligence knowledge, intelligence base if you like, gave knowledge which greatly reduced that fear of a surprise attack.
"And, as the Cold War developed, more confidence developed that the other side was understood, and that helped manage the situation and was a key reason why we got to the end without a blowout."
The one thing the spies failed to predict, along with everyone else, was of course the end of the Cold War itself.
MI6: A Century in the Shadows
is a three part series for Radio 4.
You can listen again to the second episode
Heroes and Villains
via the BBC iPlayer.
The final epsiode,
, will be broadcast on BBC Radio 4 on Monday 10 August at 900 BST and 2130 BST.