Update on the post above:
I have calculated my version of our electricity bill, and it shows a significant difference to TE bill. In fact I think it displays a large measure of incompetance. I am still in credit, I think, but far less than they have shown.
Their total for payments made is for 14 payments, which is correct, but includes the latest payment made on 5 July when their bill is calculated on 2 July, which would make 15 payments.
Their calculation of cost of energy used is for 13 months which is covered by our first 12 month contract and 1 month of our renewed contract – but they have calculated at a single set of rates that bear no resemblance to either contract.
I’m not sure whether to write back and tell them of their errors or just to sit and wait for future bills. We should still be in credit but only c£140 not £650. My wife says we should just leave it if they think they owe us more than they do.