Thank you again @newapollo.
I understand about the mid billing cycle thing, although I don't agree with it as I see absolutely no reason why, having been given the required 30 days notice, Virgin can't adjust the final bill to disconnection day (as it states on their website that they will do). After all, they're the ones who disconnect, not me, it's not as if I can access their services after that date.
However, what I do not understand, and the question Virgin repeatedly refuse to answer, is why they will take a further payment several weeks after the service has been disconnected. I have already paid them up to 27 Dec 24 even though they disconnected on 11 Dec 24. I understand they will make me wait for that overpayment to be refunded. But why are they then going to take yet another payment on 30 Dec 24, as one email told me they will do, several weeks after disconnection.
If it was any other company (not that I've had the misfortune to deal with any other company with such appalling 'customer service') then I would cancel the direct debit. But I daren't do that given the reports I've read of other customers doing the same then finding themselves with marks against their credit report even though they never owed Virgin Media anything, rather it was the other way around as in my case.