Well, the standard sump went back on. Performance is perfect with the ball oil pressure relief valve. Plus the spring cut to the correct length! It's sorted!
The sump is in storage. I didn't want to refit it having rebuilt an engine with less than 3K on it! If I decide to dig it out, will probably cut the bottom off to ensure it is squeaky clean. Problem appeared to be blast media that was embedded in the powder coating.
