I had similar, my electricity company contacted me and told me they thought my meter was US, as i got a refund too, they came looked at it and replaced it, recording the old and new numbers on a card beside the new one, they then took about a month working out their estimated usage based on the previous year and sent me that bill, I disagreed with their estimate stating weather, holidays, work patterns etc and they chopped it down a heck of a lot to a derisory figure which I paid... NEVER agree with their first figure as it appears to be fishing for money that favours them, simply contact them and disagree, it is up to them to prove what you have used and they can't. Plain and simple, they can only guess and estimate it.