I bought a flat in 2009 with a sitting tenant who paid £63 a week rent. When he moved out I went to read the meter and he showed me in his rent book that £4 of the rent was for the gas. For the last five years I thought he was paying the gas bill and he thought I was: the result is I now have a bill from the gas board for £1400!
You should see a solicitor urgently to check whether the arrangement set out in the rent book was legally binding, and if so whether you have to accept that it was in full payment for any gas used. By my calculations the tenant was only paying for two-thirds of the gas he used. It’s difficult to believe that no-one has sought payment for the gas used for this length of time.