You can do several things:
1) Write to British Gas. Make sure you send it recorded delivery and keep a copy. Register a formal complaint, this will be hard to ignore. Their address is:
British Gas, Spinneyside, Penman Way, Grove Park, Leicester LE19 1SZ
2) You need to allow them to investigate. If they do not or reject your complaint then you can complain to the Energy Ombudsman. They are independent and offer a free service at:
But you need to complain directly to British Gas before you can go direct to the Ombudsman.
For more information please see:
Can I clarify anything for you about this today please?