First of all you need to name the housebuilder. It helps others that read your post who may have similar problems.
It also gives those that reply half a chance of helping you!
Quite frankly, even at £100 a month it is not that high when the weather is below zero outside. I used £1.64 a day in gas over Christmas.
So lest have a guess.
1) If you are on the top floor, check in the roof space as the insulation may not have been laid.
2) What fuel does you underfloor heating use. Electricity is more expensive.
3) is the flat below you empty with no heat8ing on?
4) Does your floor have thick carpets which will stop the heat getting through to your rooms.
5) It could be that the meter(s) is faulty. Are you paying a supplier or is it part of your management charges.
6) Perhaps the heating is provided via a central source serving all the flats aka district heating and everyone is paying a high price because a few have their thermostats high and/or are letting the heat out.
It might be worth paying for a thermal survey which will highlight to you where your heat is escaping.
You can use this as evidence to get missing insulation fitted.
Complain to the regional managing director at the regional or head office. I would say that the strange £777 over three years was wrong. It should have been given as per year which is about right.
