Apparently they did, because the amount debited was £11 less than the amount shown on the bill.
Unfortunately, the even bigger mistake is that I don't actually owe them any money on the phone bill because I am paying them £30.50 per month for the digital TV Family Pack, and that's meant to include phone rental. This hasn't changed for a couple of years, at least, so I'm not sure quite what made them decide to start billing me in the middle of July. (Also, they've apparently started charging for Call Waiting, which always used to be free, as you can see on the bill, and I had been given no indication that they were changing anything.)
So anyway, I rang them up, and got through to a quite helpful chap who could see it was wrong and put a credit back on my phone bill (which, unfortunately, will probably be there forever and a day because we don't make phone calls on that line). As for stopping it from charging me in future, he said he couldn't do that, but he could put a discount of £11 on instead. Crazy, I call it.