Hi Will,
When you say return tubes do you mean the oil tubes, part no 36 in the PET, page 11? (see attached) If so I gave both a very good clean with de-greaser and watched them like a hawk after running the engine up and could see nothing coming from the area. I'll have another look just to double check so thanks for the advice.
I did at first think / hope it was coming from the return pipe so took off the coupling and fitted a new aluminium sealing ring and tightened up the coupling nice and tight. You can see that I put a small dent in one of the tubes
but it wasn't coming from there.
ATB