@Getoff
Thank you. I've no idea why I would suddenly have used so much in the middle of last summer. There was one day I left a gas ring on, very low hence not spotting it, for about three hours.
The arrears has literally just appeared from nowhere, but it's now only c£200, so by my estimate of c£200pm due, on £466 it will be gone in one month. But they won't allow me to adjust the DD at all.
I did take readings, and I submitted them, and I got emails saying they had got them. But Sept last year I suddenly noticed all my electric bills were estimated and the website said "you have never submitted a reading", and it turned out they had an old meter on my account, when I submitted readings they didn't make sense (against that meter, understandably) so they just ignored them and did a load of estimates, instead of wondering why I and their meter readers were sending in figures lower than the last readings by multiples of ten. Frankly, they're idiots.
So, they've apparently caught it all up now, which I think has skewed the figures. I'd happily pay off the arrears to get to a zero start point but I just don't trust them. And that was only on the electric, I can't understand how it could ever have made that much difference.
I'm taking fortnightly readings now to see if I can get it all back on track.