Well, as it turns out, British Gas's specialist cock-up untangling team suck royally at returning phone calls, and I still haven't spoken to them directly at any point. However, they do seem to be good at their primary job: they just passed on some results through a normal peon and the upshot is that they have independently come to the same conclusion as I have about how my electricity bills got so weird. That's a relief; I expected the hard bit to be convincing them that I didn't owe them 1500 quid.

They tell me it's now all in hand and I can expect a more sensible bill within ten days. I confess I'm not holding my breath on the ten days bit, but it sounds basically hopeful apart from that.