Overpaid electricity bill: options?

crayola

Hi all

I recently moved and have utilities supplied by British Gas. So far we've had one leccy bill which I paid quickly as the due date was approaching, only to realise they'd estimated quite a bit higher than what we'd actually used (stupid not to check first I know, but I was in a hurry to sort pre-xmas finances an was worried I'd forget so did it online at work)

I'm not actually sure what I can do about this: if I give them the true meter reading, will they allow me to use the over-payment as a 'credit', and if so, can I ask to put it towards (or in this case pay, in full) the separate gas bill which I have not yet paid?