As its getting colder and wetter I've noticed when I have the heated rear screen on, and therefore the mirror heaters on, the passenger side mirror doesn't clear. My first thought was a fuse must have gone but looking behind the glovebox it looks like there is just one fuse for both mirrors and the fuse is fine.

Anyone have any ideas?

Check the wiring inside the wing mirror. When stopping mine from clicking I noticed the two connections had corroded and turn white. I left mine as I couldn't find my crimps to remake them

Okay, will have a look, is it easy to pop the mirror glass out?

Have a read of this.

http://fordfiestast.co.uk/ubbthreads.php/topics/226773/How_to_Solve_Fiesta_Folding_Wi#Post226773

Explains how to get it out and fix it clicking (if it happens to yours)

Thanks will have a look. I don't have the powerfold mirrors so no clicking here!

I'm sure the side mirror heaters only work when the Front screen heater is on

Yes that's correct.

It's the rear screen the mirrors come on with, not the front.

almost certain its rear screen, will look in the manual tonight. Every other car ive driven is rear screen but none of them had a heated front screen...

It's definitely the rear screen