Jonathan Pearson wrote:

I'd contact the regulator - I'm sure they would take a very dim view of this
sort of underhand practice!

Why is it underhand? Price change readings are almost always estimated;
it's perfectly standard industry practice. To demonstrate malpractice
you'd have to show that the estimating algorithm being used was unfair.

Over the dates given by the OP, it was very much colder during the
second period then during the first, so it's entirely plausible that the
daily gas consumption would be much higher during the second period.
But it's just not worth getting worked up about, because any error only
applies to the difference between the old and new prices. I calculated,
roughly, that if the OP had had an even rate of consumption, the bill
would have been about £1.40 less, even after adding 5% VAT.

In any case, IME, if you can supply your own more accurate (and
plausible) figure, it will be accepted.

Oh, and the OP seems to have a metric meter.

--
Andy