Just an update. I checked sprockets and they were both OK. I put a dial gauge on the crank both bare and with the rotor, that was OK. Mainshaft was ok but difficult to measure owing to splines, but measuring to each spline gave no bend. I put on a new duplex chain and replaced the clutch hub bearing. I checked my records and the sleeve gear and the other mainshaft bearing had been replaced, as had the layshaft bearing. The sleeve gear bushes were not replaced. When I tightened the primary, as per manual then retightened rear chain, the primary went tight. The gear box is not moving so the front sprocket must be being pulled back, therefore tightening primary chain. What is the most likely culprit, sleeve gear bushes, mainshaft/clutch hub spline or what? Should I even worry and just adjust chains with a bit of extra slack?