PN I believe you are correct. Before about 1998 it was very difficult to find out what R&D was being conducted. Then it was all put on a database, so (in theory) you can now search by key word, and avoid reinventing the wheel. If I recall, a Group Captain in Main Building ran an Applied Research Package into ANR in the 80s, but his remit was for RAF Harrier only. They had difficulty integrating it into the helmet and work ceased. No process existed for others to pick it up. In 1994 I had to phone round and stumbled across this chap, who passed me all his old work. It transpired a company had carried on with development, and owned the IPR for what was inside the earshells, but not the circuit design itself, which was ours. The first production version used battery boxes, then it was fully integrated into aircraft power and comms - with the helmet becoming a comms system LRU as well as AEA.
Generally speaking, Analog ANR is aircraft type and, often, Mark specific. Anyone selling a generic aircrew ANR is having a laugh. The only exception I came across was Sea Harrier and Sea King AEW could use the same one, as the damaging frequencies just happened to be the same, but from a different source. MoD's Digital ANR was designed by 1999, the idea being a single helmet mod, and you blew the EPROM for whatever aircraft you were using that day. Analog met the old 85dB(A) requirement, DANR the new 75dB(A).
The MoD-wide noise protection business case was submitted on 1st August 1998. Don't know what became of it, but I know alternative technology is now used which wasn't physically small enough at the time.