I have finally got my clutch to work by extending the M/C push rod about 10 mm and*removing virtually all the slack.
There is no air in the hydraulics and no free play in the clutch mechanism in the bell housing.
I still think that this is fixing the symptom rather than the cause as it worked before disassembly (but with a tiny bit of drag).
I suspect that the pressure plate is not lifting evenly and may not have*been helped by me compressing it fully in my press when I couldn't see anything wrong with it?
Anyway, hopefully it will keep behaving itself.