I've just rebuilt my engine with new big and small ends new valves and guides rebore and new piston I've set it up according to the book but there doesn't seem to be very much compression Any suggestions?
My first thought is to start by checking that both valves are closing.
You have some valve overlap to deal with, also rings that are not sealing very well. Try putting some oil down the plug hole, this will help it seal. Once started you will very soon find the compression rising.
I put a new piston and rings in my B31 as the existing +.020 bore seemed OK. I was disappointed with both compression and performance and had a look inside after 100 miles and found that the rings were not making good contact all around the bore. I managed to find a NOS +030 Hepolite piston and rings and had a rebore. Compression was improved a bit but now after about 700 miles the rings are bedding in and compression is about as good as I remember it should be from my youth (allowance made for some weight gain in the intervening years). I assume your valve lifter is properly adjusted.
Did you get the valve seats cut to take the new valves? They might not be seating/sealing correctly after fitting new guides if not. Also check valve timing.