Can anyone help please?

Hello. Ok, here's my problem. I used to rent with 3 other people. All of the bills were in my name. I have now recently bought a house so I cancelled all the bills. I now have a creditor calling me for my gas bill. What they are telling me happened is they came door to door trying to get people to sign up for a 5 year fixed rate contract for gas. Apparently one of my roommates answered the door and thought it was a good idea. Well now they are telling me that I have to pay for the next 5 years. I don't heat with gas in my new home. They admit that they don't have my signature on anything but still expect me to pay. I feel that it shouldn't even be valid because the account is in my name and someone else signed. Do they have grounds??