HI Ormer, Got one of these myself (1982 era) so know a little bit about them, take the starter off and check that the gear on the end of the shaft is not worn or broken, then check that the shaft is actually turning and not just the motor, in the HOLE! that removing the starter leaves you will see a gear that mates up with the starter motor end, this you should also check to see if there is a tooth missing or it is loose, grab hold of it with your finger tips and give it a whiggle to see if it moves side to side, if not then take out the plugs and put it in 2nd gear and using the rear wheel turn the engine over to get the gear to move around so you can see if any of the teeth are missing, mind your fingers in the gears as it will take the end of your finger off before you know it, you will need a really bright torch and the patience of job but this is the only way I know of checking if the problem is with the starter motor or the actual gear itself. Hope that helps a bit and let me know how you get on if you would.