Problems with outstanding Gas Bill

Daddy_Pig_2

We moved into a new build property nearly 2 years ago.
After a few months we changed to a "cheaper" supplier and gave the final reading with our details to the original supplier. As we received no final bill we contacted them on 2 further occasions giving the same information. It subsequently took the supplier 14 months to send us a bill.
After being recently being made redundant I am not in a position to pay the bill and have seen something about the code of practice for accurate bills where suppliers cannot invoice for gas supplied over 12 months ago.
We have written to our supplier advising of this but they just sent us another bill for the same amount.

Any advice would be greatly received.