I have contacted British gas and they say we have to come to an agreement about the amount of Gas I have used.
That is correct. It is for British Gas and you to both agree something. If BG demand that you pay their first attempt at an estimate if you are unhappy with the amount, then they would be being unreasonable. If you say that you wont pay anything, then you are being unreasonable.
You should take some meter readings so that you can also estimate past usage. You might say, for example, that the usage for a year will be about 52 times the usage over a week. You might suggest that they wait until a full year's usage has been recorded to estimate the last year's usage. Hopefully something sensible can be agreed.
You do not have to pay their bill if you feel that there is a better way to estimate usage.