Did you keep meter readings from when you first moved in?Firstly, DO NOT pay the bill, take a reading of the gas meter now, phone your supplier,give them the reading and ask for an updated bill.
I've just had a 6 month battle with British Gas, my 1st bill was over �500,(the actual bill should have been �44) they threatened to cut off the supply,send in the bailiffs with police escort and welcomed me as a new customer.
Many letters and much angst later it has just been resolved, energy watch helped a lot.Stick to your rights!!