Don't go anywhere near British Gas. They are very difficult to deal with and customer service seems to be low on their list of priorities.
I moved my Gas and Electricity from BG to a 'green' supplier last June. There was a balance owing to me on the changeover date so I left my direct debit live so that BG could refund the overpayment to my account. A month later I found that they had continued to debit my account and when I called they said they knew nothing of my change of supplier, despite the fact that they had received the final meter reading from the new supplier. Eventually I got put through to one of the rudest women I have ever spoken to. She said that I would have to wait for my money until it had been "escalated", whatever that means, and that I might have to wait up to 4 months. I eventually received the refund last week.
I'll say it again, DON'T GO NEAR BRITISH GAS!!