Well, the title explains all really! This morning it was quite damp and the right hand mirror didn't heat up and clear the condensation. The adjustment still works fine, as does the left mirror. 

 

Any ideas? The only thing I can think of would be if I managed to damage it changing the indicator bulb! It's on warranty still but it's a bit of a hassle to get it to the dealer. 

I'd think that your first step needs to be to prove your working on that assembly has not disturbed/damaged the heater cable.

Take the mirror out and you can see if its connected up and whether you have power

Yeah I'll try that.