I assume that you have a Combi boiler (No cylinder in the airing cupboard?)

If that is the case then your central heating system has lost its pressure, like its saying.

Underneath the boiler will be a bank of pipes going into it. There should be a braided hose joining the heating pipes to the main cold water feed. This will have a Tap on it and a pressure gauge attached. Turn the tap on till the pressure gauge reads 2 BAR. Then start on the ground floor with the heating off and vent all the radiators moving to the upstairs ones after, you may have to top the pressure back up to 2 BAR a few times.

Once all the radiators are full, check that the gauge reads 2BAR and then try turning the heating on as usual

Let me know how you get on mate