I just got my latest invoice from TalkTalk. It shows a credit of £1577.67.
I have no idea why they have suddenly decided to be so generous, but perhaps it's to reward the loyalty of a very long-term (20 years) customer.
Or maybe they just messed up.
So I've told them about it, and they're looking into it. If that credit stays in place, it must be that they did it to reward me. I'm not complaining.