The most common problem with the rear brakes is the carrier bolts seizing resulting in the inner pad wearing away and ultimately if not caught in time the pad can slide out of the calliper and either jam between the calliper and disk or drop out all together so you end up with the piston acting directly on the disk.
Pete's problem of severely worn outer pad is rarer and normally caused by the inner pad rusting onto the pad holder so when the brake piston pushes against it only the outer pad wears.
Pete, the key thing is to use a file to clean up any rust where the pads sit and clean off the little silver
Clips - if you don't have any clips get some, and put some coppereaze on both sides of the clips, it will slow down further rusting under them and help the pads to slide towards the disks.
Regards,
Julian Voelcker
07971 540362
[Email address removed]