Bongo Fury

any suggestions? I have a oil leak from the oil cooler area possibly back of head. I have done all the usual stuff changed oil filter twice now have a bosch one fitted still leaking,thought it might be the oil cooler but now think it might be further round the back the only problem I have is to see where exactly its coming from I have to strip the engine to a point that it wont run so finding it hard to detect leak, is ther any common leaks from that area that i might have missed.Should have also mentoned that its a p reg 2.5d runs great no overheating no water in oil only have to top up every 3-4 months.
Thanks

sure its not just rocker cover gassket

Found problem there is a small allan key bolt on side of head behind exhaust gasket plate it just needed a tighten up easy fix Thanks to my mechanic Les.