ok, The frost sensor is indeed in the Passenger's door mirror. It's wiring is with the electric mirror wires. I found the connector, opened it up and there's no corrosion at all. So, how much is a replacement sensor? £85!
as it's not sold seperately from the whole door mirror assembly!

So That's not going to happen!

Anyone got a busted up passinger door mirror I can have the sensor out of please?