I had received a rather worrying letter to the effect that I hadn't paid any bills at all since I moved here (this to that second account number; right name and address, slightly misspelled postcode) and I started looking into this and wasting a fair amount of time on the phone to them as there had been also a problem with over-billing and possibly a case of mistaken meter being read, after which they credited my account for well over £400 (not admitting to anything, those companies never do, 'good will' blah, blah..), had an email from FU telling me that "the details in the first account number were correct" (this is the one I knew about and paid every month) but the reading on the second account number were the correct ones.
I have now received yet another bill to that second account number now... now well over £1500 ‘overdue’ according to this. And the readings seem to be identical to those in the ‘real’ account number. More hassle in my future over this, no doubt. And I’m charged way more for the gas supply in this tiny 36m studio flat than we did at the old four bedroom, two level flat in KTR.. what gives?