I'm on a fixed tariff – why has my Direct Debit payment changed?
A fixed tariff sets the price you pay for each unit of energy that you use for a given period; it does not 'fix' your annual energy bill or your Direct Debit instalment amount.
So your fixed tariff will protect the unit price you pay from any price increases in the energy markets. But there are other reasons why your Direct Debit amount can change. Perhaps your energy use has gone up or down, or your account has gone into debit or credit.