Truth is I don't know much about the RAF and Enigma but I did do some research on the naval war and as far as I could tell practically all, if not all, the decrypts are at The National Archives (in ADM223).
In the naval war Enigma started to become operationally useful around May 1941 at the time of the loss of the Bismarck and became more and more useful as the war progressed.
Hinsley says in the book (I have four volumes) that they were given unrestricted access to all British intelligence records in all archives.
It's interesting reading Roskill (Royal Navy in WW2) published before these revelations and he just refers to 'special intelligence'.