unpaid gas bill

by marie
(newcastle)

I lived in a shared house 5 years ago. at the end of the year we informed the gas company that we were moving out end of July 2007. we paid some of the final bill (which i think was in my name) and then one of our flatmates said she would pay the last bit because i was on holiday abroad. i have never heard anything from the gas company or the estate agent since, not one letter, not one phone call in 3 years until last week when a letter arrived at my mum's house from a debt collector.... so i can only assume the bill didn't get paid, i have no idea how much it was even for now, about £100 i think, i don't even remember which company it was through or anything, after a few months i didn't bother keeping any of the old account details.

Am i liable to pay this if they have never once even been in touch and have instead just passed it straight to a debt collecting agency, this seems unfair and is going to have a negative effect on my credit rating and i have just started working as a teacher and was going to try and get a mortgage in the next year or two.....

any advice would be so helpful

thank you