try vagparts or your dealer, but you'll pay a premium for the genuine parts.
the heated and electric doesn't matter for the case, only for the glass. I had to buy a new glass for my passenger mirror on the day I picked up the new car as I cracked it when removing it to fit my S3 mirrors. Cost me about 35 euro from what i remember. shouldn't be any more then 20 quid for you.
It was a while back when i bought my S3 mirrors, but the genuine ones should be about 105 pounds each.