A couple of summers ago I had an outrageous DWP bill that was almost triple what I used to incur. I was advised to pay it upfront and look into it as they'd simply credit my account should there have been some error.
I took photos of my meter reading and kept track of it to see if it was running incorrectly and all it showed was that everything was normal. They tried to say I wasn't reading the meter correctly which drove me nuts. I even got one of those metering devices that you put between what you're testing and the wall outlet and got normal readings as well on all the major appliances I have.
Eventually they sent out a meter reader to double check on things and it turns out the first guy misread my meter. You see, there are 3 banks of meters in a long, dark hall and mine is with the last group furthest in. Maybe it's something as simple or similar to that.
All the best,
Nonoise