No idea of actual amounts, but it really depends on the size of the rooms, type of building and location in the building of the flat.
For example, an energy efficient new build house will be cheaper to run than an old style victorian flat with big windows and high ceilings. smaller rooms heat faster and a flat in the middle or towards the top of the building is generally warmer as it receives heat rising from the flats below. It also depends on how many walls are exposed to the outside and how many are shared (by people who heat their homes).
All things being equal though, the flat should be cheaper.