Frister and Rossman Cub 3 whirring but not moving?

The motor whizzes, but nothing moves. Looking down from the top I can see that teeth are missing on the large wheel at the end that would connect with the rubber wheel for the bobbin winder. Trouble is that I can't get that end of the casing off. I've undone two screws and unscrewed the plastic drive, but something is still holding it on at the top. Any ideas? Am I just going down the wrong route anyway. Thanks