£200 here.

We'd been paying the same amount for quite a while when my husband decided to have a look at our energy account. We found out that we owed them in the region of £1900. In the past we would have had a letter in the post about that or at least a lettter at the end of the year telling us if we used more or less on our energy bill and then it was adjusted. But as everything is online now that is not done anymore. We had no reason to doubt what we were paying and were happy enough with it, so no reason to check.

I wonder how long it would have taken them to contact us and how high my arrears would have been by then.

So we paid off a lump sum and the rest is now added to the monthly bill, hence it being so high.