Normally you build up credit in the summer & debit in the winter, but with this summer the way it's been, presumably you haven't been building up the necessary credit - so if the payments stayed the same, it wouldn't cover your winter use.
But, yes, you should have been told -I usually get a letter saying it's going to change.
We had this, but for our gas bill, which i knew was way to high. So when we got our last bill which also was the same time as them changing
our meter. I phoned with an actual reading which reduced it down to about £11 per month from £26 per month and also cleared the outstanding amount on the bill and left us in credit with them.
So i would just suggest next time you get your bill just check the reading is correct, because ours was almost double what we were using.
I did this with both gas and electric and have saved us nearly £30 per month.