GTAT, I read something similar in the press. There are rules over the repayment etc. I suggest you contact Ofgem and pose the question to them.
I would also try and speak with your neighbours about their actual gas bills so that you have a ball park figure to compare with Scottish Power's estimate. Ideally you will find the identical house with an identical family ie wife at home 2 kids ages the same etc.
Speak with the Families Association; they will be well placed to give good advice.