Having had problems with it before, I have left this until last minute/worst weather to fix. I have just parted with

Sounds like the new switch is defective as well..... can it be checked with a multimeter? I have not taken one of these switches out before so I don't quite know what to expect - due I assume the rear demister is actually controlled by a relay, and the switch just powers the coil of the relay?

The indicator on the switch could be powered from the output of the relay and the relay is stuck on, switch on the heater and pull out the relay to see if the indicator goes out, if it is the relay prise open the sides and clean the contacts with some thick paper soaked in WD40, you should hear a click off the relay as its switched on and off.

Ok, sounds good , pardon the ignorance, but where are the relays on these things?

I think they are above the pedals, you remove some sound deadning trim and hey presto, trouble is I cannot remember which relay it is, it should tell you in the haynes manual or have a search on the forums. cheers - Stuart.