Hello there I'm an ex-Powergen employee and just wanted to raise a few points people may or may not understand.
Capped pricing doesn't guarantee how much your bill will be, but merely the unit prices you pay. Therefore if you use more then you will still pay more. Also these tariffs tend to mean paying a premium at first for the guarantee that those prices won't increase for X amount of time.
When Direct Debit amounts are amended the new amount will almost certainly take into account your usage plus ehatever the amount is outstanding at tthe time over a period of say 4 months. If you are unhapy with the proposed rate then contact your supplier. They will usually be happy to discuss an alternative arrangement, or allow you to clear the arrears first and set the DD to uasge only.
I read somewhere people saying that they would change suppler then pay what they owed as they pleased. This is likely to get short shrift from the suppliers and probably a visit from bailiffs.
Finally if estimated readings are used then this is made clear on the statements issued, usually in large bold print. I always found myself struggling to sympathise with people complaining about this when we later updated their readings given it is plain that the bill may not be accurate