Just an update:

I'd resigned myself to living with folding out the mirror manually each time as the repair is so expensive for what is a luxury function.
But yesterday I'd driven the car after folding the drivers mirror manually. Parked up and locked the car, and the passenger mirror folded as usual.
When I got back in the car I noticed that the drivers mirror had folded too.
Started the car and both mirrors folded out as they should!

But now it's back to the drivers mirror not folding.
So it seems it's some connection issue rather a motor failure.

Anyone have any diagrams on what's behind the mirror plate and how to remove the mirror to get access?

I assume I'd see the motor and connector behind the mirror plate if I could take it out?
Hopefully it may just be a corroded / loose wiring connection to the motor.