Your liability or otherwise for the gas bill depends upon the terms of your lease contract. If the lease contract identifies the housing association as being responsible for the gas then you not obliged to do anything. If the lease contract identifies you as being responsible for the gas then the contract of the gas supplier is with you and not the housing association. In such case, you must notify the gas supplier and pay for the gas for the previous four years if it has not already been paid. If the housing association has being paying the gas bill in error, then you are obliged to reimburse the housing association for the amount paid to date. Failure to do so is a sin.
And Allah knows best.
Mufti Mohammed Zubair Butt
Chair, Al-Qalam Sharia Scholars Panel