tried replacing the oil pressure switch on the top of the oil filter housing, It's hard to spot but mine went on the old white valver and i had no idea where it was coming from until i put it on a ramp and prodded about all around the oil filter, then bingo there it was dribbling oil everywhere.
Might be worth a look, if it is they cost about £3 from GSF and take all of 5 mins to replace.