Ian, if you look at the mounting of the motor you will see a nut and bolt. Slacken the bolt and the motor can be moved. This should enable you to fit the belt. When you come to tension the belt up just apply enough tension to grip the belt, so that it can not be easily slide off the pulley. If you over tension it you could wear the motor bearings or belt prematurely.
Hope this helps.
Mike
May 2005