Hi there.....I've had an ongoing problem with British Gas too.....I received a 'bill' last year that showed that my electricity was �600 in credit! Obviously I queried this with them and they said that it was right and that I could expect to receive a cheque from them.
About three months later, and after receiving my next 'bill' I took the matter up again and spoke to I don't know how many people at BG to confirm that I did indeed have a big credit on my account. Once again, they promised to send me a cheque which to date, I still haven't received.
The reason for this credit was the fact I had been paying too much on a metre (just like you) and I had been paying too much for the entire time that I have lived in that property.
I have given up chasing BG as I've already spent hours on the phone and been passed from pillar to post. In fact, I must be due another 'bill' about now....so maybe this will prompt me to chase again if I can muster up the energy (!!).
Good luck with them and if you can, let me know how you get on.