I've had a look at the diagrams James, each mirror has two motors one for up/down another for left/right.

The thing is they are not fused seperately so as one is working each side it cant be a supply problem.

The door switch itself is a very complex multi contact set-up to effectively run two motors forwards and reverse etc.

I suspect either a problem with the switch itself or maybe some broken/bad connections in the door loom.

Sorry I cant be more help.

.

.Bob