What you need to do is read the meter yourself and ring British Gas with that reading. If you have overpaid any money, it will be taken off your next bill. For example, if you have overpaid by £50, then British Gas will adjust the bill downwards by £50 so that the next bill is correct. You can do this every time they send you a bill, so that you do not get billed for more than you owe, or you can pay based on the estimates they send you and get it adjusted each time they read the meter. They are only obliged to read each meter once every two years, I believe, and can send estimated bills in between those times.
You cannot get accurate readings for the previous months, as the meter was never read at that time. If the current bill is based on an actual reading, whereby someone came and read the meter, that bill will be accurate and you will be credited back any overpayment, as I described above. Estimated readings are based on the amount charged for the equivalent quarter in the previous year; eg the estimated bill for May to August 2011 will be based on the May to August 2010 bill, with adjustments for price rises taken into account. If someone else lived in your property at that time, and was a higher or lower user than you are, you will either pay more (if they used more) or pay less (if they used less) than you.
This information applies to both electricity and gas, where estimates will be sent as above.