The doctor actually rang on time today and said she didn't know why we were being charged and said she'd get the secretary to follow it up, and sure enough a couple of hours later I get a phone call! The GP's secretary told me that they hadn't requested a private referral and asked to see the letter the clinic had sent me so she could ring them and see what's going on. So of course I dropped the letter round to the surgery straight away and I'm waiting with high hopes - or a satisfactory explanation!