The Empirical View
My researches begin in 1938 and mainly concern the Valley Works, Rhydymwyn and the connection with the initial R&D on the Atomic Bomb. It also covers their postwar disposal.
All of the relevant files were released in the 1990s to TNA and what is now DEFRA records at Alnwick. We have copies of most of these. There are no secrets we are aware of but when we question some of the wartime workers they are adamant that they signed the Official Secrets Acts and they are sworn to silence for ever. The questions we wish answers to relate to everyday events and their social life but the answer is still no.
It appears that there was a later Cold War use of the site from the 1960s onwards but with a little persuasion MOD, the Treasury and the Cabinet Office all released files up to 1980. Very interesting files concerning the Bank of England gold and other now redundant contingency plans.
My conclusion is that with rare exceptions you can access information and use it publicly up to 1981 under the 30 Year Rule but there is a problem with whether potential witnesses will accept this.
Hope this helps.