Pixie09:
My goodness. There are multiple issues going on here. You need a very systematic step by step approach to sort this before it gets out of hand.
Lets start with what you should have done immediately you left your old address so that you can work out what you did or did not do and therefore still have to do.
What you should have done is to have taken a closing meter reading and phoned the customer service line and announced you were leaving. The adviser would have done a workflow which would have asked for the closing reading and a forwarding address. That's about all. Possibly they would have tried to sign you up for your new address. There would have been no need for rental agreement proof of new address at that stage.
OK, You didn't contact them until March. Was that by telephone? Did you tell them you had left? Did they ask you for a closing reading? Did you tell them the closing reading? Or did you just write?
You need to look at the dates on the £700 bill. It should be a closing bill for service up to March 2009 with the closing reading you passed in March 2009. OK, you need to tell me if its not and if it includes an estimated (or even actual) recent reading then the account has not been closed. In that case it may be one person's word against another regarding the March 2009 contact. If that is the case I would strongly recommend you attempt to obtain the current reading. The worst case scenario would be liability for supply from Feb 2009 to present and not too much gas unless the heating has been left on high.
Sound out Npower about your alleged Mar 2009 contact and note what they say.
Post their response and date and reading details from the £700 bill.