I'm not sure but on the back of your statement should be a calculation explaining how they convert your reading to the kWh that are used to charge you.
a metric meter will be something like:
Units x Calorific Value (39.6225) x Volume Correction (1.02264)
Divided by conversion to kWh (3.6) = kWh used.
You will also have a metric conversion factor as you have an imperial meter.
.
I recently used 14 metric units (over 1 month)
This converted:
14 x Calorific Value (39.6225) x Volume Correction (1.02264) divided by 3.6 = 157.57
This was charged at 6.86p = £10.81
This was a low usage month compared to average.
Compared to my usage (157.57 kWh) over 31 days you would have used 447.3 kWh.
You have used 3 times this amount.
I have my heating turned off and the hot water is on demand (with a new boiler < 2 years old).
Gas cooker and electric oven.
HTH
Sparky.