The short answer is remove the galvanised back from the main case and replace the belt. This is accomplished as follows.
Remove the drier lid - two screws at the rear, and slide the top to the rear until channels in the top disengage from their retaining screws. Pull free the spade connectors connecting the main loom to the the white block on the rear of the case.
Lay the drier face down, with a couple of cushions/wooden blocks positioned to protect the rotary switch. Remove the circlip and two washers, which hold the drum spindle into its bearing. This model has two belts, one a small one driving a fan. This small belt must be removed to gain access to the drum and drive belt. To remove the small belt, undo the single screw holding the lower white plastic belt cover from the rear of the case. Unclip and rotate the cover free of the rear of the case. Remove the small belt. Remove all the retaining screws holding the rear of the case to the main case. Do not remove any screws other than those on the extreme bottom and edges of the case. Lift the cover free (you may need to disconnect another electrical connection. Loop the belt over the drum and the motor centre spindle. Fit the black belt tensioner pulley over the upper spindle and pass the belt under it.
Replace parts in reverse order. Ensuring that the new belt is running on the marks on the drum left by the old belt. Test and use. (I found the black belt tensioner pulley, free of its spindle, lying in the foot of the case.
Ian Ainslie, October 2006