I am not an expert in these matters I am having my own problems with electric suppliers.

The previous tennants
are responsible for bills upto when they ended
their contract with British Gas albeit 10 months after they have left.
I would write to BG (recorded delivery is helpful, in case anything goes further) Explain that you are the new tennants from (date) your gas supplier is Swalec and give them copies of your gas bills to prove this and the meter reading when you moved in. I would also give the previous tennants forwarding address and request that any corrospondence about the unpaid bills be addressed to them. Also request that as you have proved you are not responsible for the outstanding amount that they do not bother you again causing you unnecessary stress.
Don't forget to send a copy to the Solicitors aswell (recorded delivery) Keep all your records together letters, bills etc
I hope this helps it is just what I would do. It is not professional legal advice
