Hello Osbear,
Undo the screw in the stop motion wheel, you will notice that the stop motion wheel has grooves all the way round or should have, looking at the stop motion wheel fron the end face on, get a large screwdriver, put the blade bit on one of the grooves on the left hand side, with one hard thump on the handle of the screwdriver it should free off the stop motion, when you have turn the stop motion wheel just hand tight and tighten the small screw again, e-mail me below if you have any problems.
Ken
drivewithken@blueyonder.co.uk, October 2008