I too have a similar tale of woe.
Left house in 2006 fully paid up, as of two months ago, they were still chasing for an erroneous debt.
My final correspondence with them invited them to issue court proceedings (I'm a solicitor) and advised them I would be interested to see how they would justify pursuing me over a reading that they admitted was incorrect and entered by one of their own staff before we left the property. Still waiting to hear back… 😉
They're bully-boy tossers – I particularly hate the way they send letters threatening bailiff visits from supposedly independent companies, which are actually Npower subsidiaries.
I'm expecting this to pop up again in 6 months, as they seem to diarise it to chase every 6 months, and when they do I shall issue proceedings for an injunction against them following Ferguson v British Gas.