Well, maybe in hindsight I should have saved that money towards an engine o/h instead of doing a panel.....
Word is good. No metal in filter (or oil screen in my case), OPR valve checks out and the spring is good, but was probably set too low. He's now turned it up, test run it thoroughly and it all checks out and works! Famous last words - only a prolonged flight will give the final answer, but it's looking a little bit promising. Now work has come in the way and I can't pick her up until end of next week, so won't know until then.
LATEST UPDATE: We're flying again. All seems to work.