My heater will only work on setting 4 and the air con comes on.  My garage tells me its a common complaint and they will need to fit a new resister.  Has anyone else experienced this?

The Mk1 Fabia's suffer(ed) with this a lot. New resistor should fix it for X amount of time.

Had this on one of my previous cars and just replaced the resistor and it was fine after that

as above, fitted it myself thanks to a post on here and cost me all of £14, it was the ONLY thing to go wrong in 7.5 years of my mk1

Yep sounds like a resistor