I would imagine that if interviewing personnel reluctant to talk about their experiences because of having signed the Official Secrets Act, you would need some sort of official letter to convince them it's OK. I presume that's what happened when all the Enigma stuff came out in the 1970s.
The original revelations by F.W.Winterbotham came in November 1975 and were with official help and approval.
There are occasionally special cases for unrestricted access, Margaret Gowing had it for her trilogy on the Atomic Bomb, Hinsley for his HMSO books on intelligence and the SIPRI authors also have had this privilege concerning a number of publications. I believe institutes such as RUSI probably benefit from such special treatment.
There also appears to be be an arrangement where certain authors are given access to files just before their appearance in TNA. Peter Hennessey probably benefitted from the early viewing of the War Book before his last update of a previous work.
So, finding someone with an inside track who may have already seen the relevant TNA files may save you a lot of time