Sources of Information
The US National Archives and Records Administration, on the University of Maryland campus also houses the CIA Archives. If you happen to be in the Washington DC, take a day to visit the Archives (there's even a free bus from down-town DC). They will appoint a researcher to assist you and if you mention 'CIA' you will be wisked to a secure part of the building where another researcher will sit with you in searching the extensive records and seeking to declassifiy material.
As I found out, the FRUS Series (Foreign Relations of the US) contains a lot of British documentation - including material embargoed at Kew such as operational intelligence matters from the 40s and 50s. The latest material I obtained on UK involvment in SE Asia was c 1969, but I ran out of time to seach for more.
To register with NARA is free; p'copying is free and they let you take a digital camera to copy docs. The CIA archive will print reports for free, and then every sheet is then declassified by hand and given a smart stamp.
CF