Thanks for the replies
I took the whole side mirror off and stripped it down (lots of parts!) Right to the motor used for folding.
Inside the motor assembly are a few cogs, these are small and inaccessible really. I think the cog on the motor shaft is no longer secured, and just spinning on the shaft.
Definitely not worth trying to repair, even if I could find exactly what the problem was.
So I unplugged the motor connector which for future reference should actually be accessible by just removing the mirror cap, I'll try and attach an image.
So at least I still have electrically adjustable mirrors, just the folding does not work on the one side.
I'm going to look for a replacement at a breakers
Thanks all
Photo with mirror 'cap' removed
The motor connection is on the left of the photo, pull the grey rubber bung out and you will see it
Edit: can't seem to attach an image - apologies