Do nothing.If you dont have a current billing company then you cant ask ,say, British Gas to bill you in future as they dont know who you are paying, or not paying now. So the account cant be finalised. I suggest taking monthly or quarterly readings work out the costs from gas tariff web sites and put the money in a building society.If a bill does finally appear they will not charge interest and must base charges on rates applicable throughout the period.Do you have the last reading when last billed..