Thanks for your reply, @
Anasa_EONNext. I've not yet heard back from your complaints department, but I'll report back here when I do.
Meantime, it would be so much better if all this had been explained to us when the EBSS started, so that I would have been ready to make the adjustments myself. The "What's happening to my bill" page on your website doesn't say anything about this either.
The email that I received is obviously a generic message to deal with a situation where someone's DD is lower than their usage. It makes no mention of the EBSS being stopped, which is currently going to be the most likely reason for a shortfall in the DD amount. Just an extra paragraph in the email to explain that after the EBSS, you expect customers to correct the DD amount themselves - that would have helped.
Anyway, I will go in and make the changes, make a one-off payment and put the DD back to the amount it was before.