PN - you beat me to it. I was going to suggest that all the conspiracy theorists should read Chris Andrew's excellent (and official) history of the Security Service, know by lesser churls as MI-5.
Having said that, a relative who was a senior 'wheel' in the Chevaline development in the 1960s has categorically stated that they had been instructed by the Programme Director not to pass any sensitive 'special' material to Dennis Healey's office, becasue of Communist links. But, as Sir Humphrey Appleby stated:
The Ship of State is the only ship that leaks from the top!
Edited to add: Fellow PPruners should take with a grain of salt anything written by either Chapman Pincher or Peter Wright. Both were fantacists who did the public and political image of the intelligence community grave harm.