This is a scam, ignore it. Over 1 year after I left a flat I got a similar letter from a debt collection agency - it was sent to the flat and forwarded to me by the new tenant. I still had the paperwork showing the bill was paid. I called the debt collection agency and they had no information about meter readings, dates of bills, or amounts on the bills, they just said I owed some money. I did a Google search on the name of the agency and found many people with the same problem. The advice given was to ignore the letter. I suppose I should have taken it to trading standards.