EDF Energy/Advantis hounding me

scoobster1

Hello, I'm in dire need of advice and I'm looking for the quickest way to end this headache:). For the year ending June 09 I rented a HMO with 7 other students. Being young we assumed the house was completely electric. As it turns out, it was gas and electric. Both supplies being provided by EDF Energy- they sent us the electric bill on time but we never received anything concerning gas. EDIT- If I was never aware of the gas - I only agreed to have an electric bill in my name. How am I even liable?!
About a month ago I received a letter from Advantis(the first of MANY!) claiming I owe EDF the gas money for the year we were there, and the following year. I've explained that the tenancy was only for a year but they refuse to listen. I've been getting constant letters and phone calls(EDF don't have my number so how do they?)telling me to pay up or prove I'm not responsible. I've managed to get a copy of the tenancy agreement for the house in question(they lost the first copy-second is in the post), but not for the following address. EDF have sent me an itemised billing thus I've worked out I owe £400(I doubt I'll get money from the others) instead of the £1000 they're expecting.
Is there anyway I can just pay this and then ignore them? I'm fed up with their constant phone calls. I've given them contact details for both that landlord, and the letting agent for where I lived afterwards to verify it was only a year but they refuse to play ball. If I still had paperwork for the subsequent address, it'd be at the family home(over 300 miles away!) which I won't be near until June.
Why can't they simply contact either landlord to confirm what I've been telling them all along? Out of curiosity, how long should you legally keep tenancy agreements? I've moved around quite alot in the last 5 years and I doubt I'd still have much paperwork for some places!

Thanks for any help:D