Hi Mr Wody
Sorry to hear about the troubles you have been having, but then again, Npower arnt the best when dealing with billing accounts. It seems from your post that you are the landlord of this property as you mention you own it and you rented this property out to a tnt which left early in january. You mention that you visited the house during the 2nd week of January to collect your mail, one letter, which clearly wasnt yours was the electricity bill in the former tnts name, which regrettably the npower agent advised you.
However common sense dictates that this is usually how any prospective new tnt or owner of a property establishes who the supplier is to that property.
Anyway as your post doesnt seem to suggest I presume this tnt paid other bills promptly and did no damage to the property and therefore I also presume you returned a security deposit, as this would allow you to provide the tnts forwarding address.
In essence any electricity or utility company has the right to chase up unpaid bills, as realistically their product has been used and not paid for, however, there is only so much a supplier can do, which usually ends up in the supplier applying for a warrant & either fitting a prepayment meter with a debt setting applied or disconnection. What I find strange is that usually their is a mass of mail which would normally arrive before this, which would have indicated that your first call, in early january, wasnt acted upon.
My advice would be to contact them again, state simply that they must act on the information you can provide, as the former landlord of the property, now actually living back in it, that Mr X left on a said date, with a meter reading, and moved to xxxxx this will provide them with an alternative address to send their mail too. Advise them that you are going to contact the ombudsman about this, and do it, just google them you will find their contact details and give them the scenario.
Finally i would be prepared to go to court, with your documentation as companies dont expect to get these things interupted by us little people, if all your evidence is sound, no warrant should be passed, but be prepared, as they might quickly act upon your information, close the tnt's acct down, open one for you and send you a bill for your true usage faster than they can say disconnection
Let us know what happens, hopefully, common sense will prevail !