I don’t know if that helps but 895 would work out at about £350 on my bill, depending on your company standing charge etc I’ve just submitted my readings for the month and used and average from my bill to work it out with the 895.
Could they have read it wrong? I had a water bill a few years ago that was three times the usual amount, and they sent someone round to check for a leak. The first hint the guy asked was to see the meter reading on the bill, apparently it happened a lot and was always the first thing he checked for.
I don't know how you calculate a bill of £28k OP. 895 gas units is about £400 depending on your tariff plus any standing charges. If you've only had estimated bills this is of no surprise and is likely to be right.