Phat Beamer: Also are these procedures followed properly as i would expect British Gas to get back to me if i didnt make payment within a couple of months latest.
Get these expectations of reasonable behaviour out of your head. You are dealing with (1) a Utilitity and (2) British Gas.
These organisations do not have a good consumer report card.
First of all they cannot charge for energy consumed more than 12 months before sending a bill, but that is not necessarily the circumstances you are in. I don't blame you for losing your records, easily done, but to fight this you need every scrap of documentation going. I strongly suggest you make a £10 Subject Access Request of BG requiring your previous bills, your customer files including record of contacts, and your account payment history.
To answer your question, if they properly billed you at the time and your didn't pay then I think you owe the money. If the latest demand is for energy not billed at the time then, assuming you participated in the account closure, I don't think you owe the money.
But you need to get the documentation to prove it one way or the other. You can also get advice from Consumer Direct.