Thanks for all of the replies.

Problem is now sorted. To explain. The engine in question was assembled by another bloke and ran a relatively mild cam. We eventually built our own late block with bigger cam, 1400 head etc.

The discussion here has made me aware that the thrust bearing on the crank is the same for both the early and late blocks. Turns out the guy who assembled the old engine put the thrust on the rear main and not the centre main.!!!!!! Hence the excessive end float.

A new set of bearings is in order and the problem will be all sorted. Lesson for young players!