I've just taken the cylinder housing bit off the top and found the problem. The piston slides smoothly in the cylinder, but the end of the connecting rod that attaches to the eccentric on the shaft is extremely tight, so the motor can rotate smoothly a very small distance now until the connecting rod touches the sides of the cylinder. The cylinder looks and feels nice and smooth, although to be honest I've nothing to compare it to so I don't really know. Presumably if I dismantle it further I should be able to make the bushing or whatever that is damaged?