British Gas & Past Due Credit Solutions problem

Shocked1

Hi,
I'm new to this so sorry if anyone has already answered a question similar to this but I am really quite worried about this. I sold a home in July this year and in that home we had a prepaid electricity meter. No gas only an oil tank. The meter was always in credit and only occasionally did we use the emergency credit.
On Saturday I received a letter at my new address from Past Due Credit Solutions, a Final Demand for £153.15 from British Gas. I dont understand this as we had a key which we topped up and if we didnt top up we had no electric. I called them on Saturday and there was no answer so I called British Gas. The man I spoke to took all of my information and looked on his system and could find no record of any debt that I owed them. Nor could he explain why there might have been a debt. He said he would have someone call me back Tuesday (Tomorrow).
So today I received a text from the DCA asking me to get in touch. I did so and was told that when I first moved to my property there was a debt added to my meter for £50.32 and that when I left British Gas in June 2011 i never gave them a reading so that is where the debt came from. I'm sorry but firstly I was with BG until July 2013 when we sold the house I never changed supplier. Secondly I was never informed of any debt at any time when we moved in to the property. It has always been a new style meter, not a token one and i have been informed that these can be updated with tariff remotely so no problems there! Thirdly why was I not sent a bill from British Gas about this at any point and why would they keep supplying electric if i allegedly owed them money. It doesnt make sense.
Please can someone help me.