Well, I realised that I can move the drivers side mirror left and right if I hold the selector switch in *just* the right position.. so that indicates a connection problem in the switch. (the N/S mirror will do the same, but slowly and intermittently, so again, bad connection somewhere)
So, here's what I did to clean the selector switch and PCB:
Carefully prise the switch out of the door card with flat blade screwdriver.. and disconnect plug.
Carefully separate the switch body.
The whole switch was very dirty, inside and out!

Separate the PCB from the base, without bending the pins:

Again, filthy! No wonder it's not connecting properly!

Cleaned up thoroughly:
However, even after a thorough clean up, I think the contacts have been badly scored and pitted, so a good connection is never going to be possible.


Anyway, so it looks like I'll need a new switch, but I thought these pic's might be useful..