British Gas estimated my bill at £1,100 when I had been away — how do I dispute this?

I was away for 6 weeks and came back to a massive bill. I have submitted actual meter readings but they are saying it could take 4–6 weeks to reassess. Anyone been through this?

The estimated reading was wildly off — they assumed I used the same as the previous occupant who had 4 people in the house. I live alone. The actual reading I submitted shows I used about £180 worth.

Do I have to pay the estimated amount while they reassess, or can I hold off? And has anyone found a faster way to get these corrected?