Bloody energy companies . . . !

My tariff agreement with nPower is coming to an end, so at their request I spoke to a nice friendly advisor. As nPower had moved me from a high monthly payment to one I thought rather low, we agreed that on the new tariff starting in September I would pay rather more. Meanwhile, we would leave in place the credit balance that had built up, to provide a cushion - we've not been in the house long, and I'm still not sure about our energy usage rate. All agreed, everyone happy, one more payment at the old rate, then the new tariff and new payment rates.  One week later, nPower refunded my credit balance unasked, then increased my monthly direct debit to a rate higher than the one agreed, beginning in August. I rang and complained that this did not seem the ideal way to do business, and was informed that "We do seem to have messed up!" - but have yet to receive any formal response.