BBC News:-
Martin-Baker knew the parachute mechanism could jam if the nut and bolt were too tight as early as January 1990, the inquest was told.
It seems illogical indeed that such a simple fault was not known by the UK Military Airworthiness Authority, aka the MOD/MAA. It seems illogical that the seat manufacturer would not have informed its major customer while informing its minor ones. It seems illogical that even were it not so informed, that the UK Military Airworthiness Authority, aka the MOD/MAA, would not have become aware over a period of 23 years.
As Nutloose suggests, the informing would have been by signal or other written means. The address lists of either would have confirmed or disproved the verbal evidence given in court. Were they presented? Do they exist?