Well managed to get it sorted eventually.
Took the oil pressure relief valve out today and cleaned all the bits and re-oiled the piston/plunger and pushed it in and out to see if it was sticking and it wasn`t so I put a small washer behind the spring and reassembled everything.
Wahayyyyyyyyy!!!! I got my original `cold` oil pressure back (about 60 psi) and at operating temp I`ve got 22-ish psi @ idle so I iz a happy man.
One thing I did notice though was I have more than my old oil pressure through the driving range (about 60-70 psi) whereas before it was around 40-60 psi so all round I`m eventually happy
Just gonna watch for any more sudden decreases in pressure in which case it`ll be a new oil pump not looking foeward to forking out for that one!! but hey, let`s just cross everything for now and see how I go eh??
thanx to everyone for all their advice