We are with Eon Next. We used to pay £350, monthly DD and earlier this year they attempted to 'ask' for over £600 per month. We refused, saying that we were willing to pay £450 per month and pay any shortfall should it occur.
Now, they've 'asked' for £813 per month and we have raised a complaint. We have explained that attempting to take such a large amount may result in a failed DD with all the negative consequences. If they insist on trying to take the large amount, we will cancel the DD, set up a Standing Order and pay any shortfall, should it occur, when it occurs.
Frankly I don't see why they should benefit from using the money of customers who are in credit (which we currently are) when we could put aside that sum, gaining some interest, until the time comes to settle the bill.
Having a monthly DD to 'avoid unexpected bills' in their words, is a con.