Thanks for the replies and advice.
The compression reading of 170 psi was with the engine hot, though only from running the engine and not after a long run. There is a manual valve in the supply oil line so not wet sumping from standing, so it could be poor oil pump scavenging or a problem with the oil pressure valve?
However, none of that explains how it lost around 2 to 3 l of oil in a 70 mile run which does not appear to have been dumped out of the breather but straight out the exhaust?
The RH piston crown had a soft black oily deposit when we took it apart last year but otherwise the pistons looked fine, but there may a fine crack? Head looked fine at the time with no apparent massive wear between the valves and guides. However, we did not make a very detailed inspection of the head, so a crack may be a possibility?
Looks like the head will have to come off again with a more detailed inspection of the guides and head for wear or cracks. Just wanted to get a better insight as to what the problem may be and where to focus attention.