"Flat is warm first thing but gets cooler as day goes on"
Yes, that's what they do.
This makes them very unsatisfactory for people who are out at work all day. The most satisfied users are people who get up early, are at home all day, and go to bed early. Such as parents of small babies and some elderly.
You need to be aware that energy from electricity costs four times as much as energy from gas. I urge you to look for an off-peak tariff, preferably one that has an afternoon or evening top-up period, and to use timers to maximise your off-peak use.
Get a diary to note your meter readings in, and a calculator to work out daily cost. You may be alarmed.
If you are in a modern, well insulated flat, with heated homes above, below and beside you, heat loss will not be great, and you can concentrate on blinds and curtains. Get an electric blanket.
If not, you will know why the flat became vacant at the beginning of winter.