That's way toooo much.
My last bill = 100 units
Total estimated charges for WHOLE of this coming year by Npower is £225.
That's for a modern 2 bed semi house. Hot water set for 24hrs, heating set to minimum 15 degrees 24hr, plus 21 degree 2hrs in morning and 7hrs in evening.

Sounds like they are reading the meter for the whole block of flats.

How many flats are there?
Are the meters remote from the flat, i.e Are they all side by side?
Or is the meter within your locked property?

Turn your gas off at your meter for a day and see if any neighbours get a visit from a heating engineer to fix their heating!!

Turn off everything in your flat and go and watch the meter to see if it is moving, try this in the evening when everyone else may be in the other flats.

.