why does the central heating come on when the thermostat is turned off alon?

The thermostat could be faulty. If you disconnect any one of the two wires in the thermostat the heating should stop. If it does the thermostat is faulty. If it doesn't, you'll need an electrician to sort out the wiring. This is the only explanation I can offer without a good deal more information.

Bob Chett.
November 2008