I wonder if there's any chance of finding out what happened here…

My electricity bill is £350. I pay be direct debit and the balance
floats between £300 and £350. Thing is, about 3 years ago it floated
between £70-£100. I do know what's happened to elect prices but I
increased the DD until I noticed it said on the bill that they would
adjust the payments automatically. But they take a fixed amount each
month and have never increased it unless I ask. It just looks like
they want me to have a big excess - if so why?

Due to circumstance I never got around to trying to find out why or
exactly when, this step-function in the bill actually occurred (hands
up - my fault for leaving it). I've been through all the past bills I
have but of course the ones that would have should the details are
lost. But I do remember at around that time the electricity meter was
changed - from a beefy black plastic rotating wheel thing, to a small
plastic box with LCD readout and blinking led!

Is there any point/chance of me finding out from their records (or any
other means) after all this time, where this large excess came from?
Thanks