Central heating works (usually) by heating water in the central heating bolier (by gas) and then pumping the hot water round the house to each of the radiators and then back to the bolier when it is heated again.
Each radiator MAY have a thermostat control (not all do) looking a bit like this
http://www.bathroom-e...0radiator%20valve.jpg
If your radiator has one then turning it up and down controls how hot the radiator gets. Note that not all radiators have thermostats.
There will also probably be a main thermostat in the hall which controls the overall heating in the house.
I doubt that sending the water round each radiator will add that much to your bill.
The two things that will effect you bill are:
1) How long you have your central heating on
2) How hot you heat the water.
Both of these can be controlled by settings on your central heating.