The drivers door mirror heater ( Fabia) does not seem to be working passenger one is fine, any ideas please, fuses seem fine, I am not sure if you can tell if they are working unless it is frosty! Thanks

A test light, would help. it should be warm to touch. more than likely a faulty mirror glass I would think if the other one is working.

What is, and how would you use a test light for the heater?

Like a small screwdriver which has a light bulb in it and a lead on it.

Connect the lead to a good earth and touch the driver onto a terminal, if the terminal is live it glows up. So pop the glass off (it just pulls out) and test the two terminals in the top left corner. One will be live and the other an earth.