Why let a gunsmith charge you 30 quid for a drop of threadlock?
Or more a new trigger unit, if he can get away with it?
The op seems to know how to set the trigger to just how he wants it, so trying a touch of thread lock is a logical next step?
He also apears to know how to fit an entire new unit himself, so it would seem he's got common sense and the brains to be able to carry out such a proceedure.
Okay if the thread lock fails, he would notice after a few shots given his earlier statement, so then I'd advise a new trigger mech.
Phantom