A quick update.
As mentioned in my previous 'STOP PRESS' post I'd discovered the ball valve had the 'kick' in the wrong direction (it was kicked out at the top). It was Annual time recently so with the cowling off I took the opportunity to remove the ball valve mount and make it right.
After a recent flight test in which I pulled many vertical lines I'm pleased to say the oil pressure stays high on an upline now, so it works perfectly.
I haven't seen so much as a puff of smoke out the exhaust either and oil usage is virtually zero after aeros so this all seems to be working very well.
I'm now looking forward to a new season of aerobatic competitions.