Please help

My central heating system seems to have a mind of its own. The radiators come on, often in the middles of the night, when the heating is turned off. They get absolutley boiling as well.
There is a normal thermostat in the house but there is another one situated by the boiler, which i understand is the one that prevents the pipes from freezing, but something is wrong somewhere as surely the radiators shouldnt get boiling hot. Also i ensure that the inside thermostat is turned to zero to ensure that is not causing the overheating but clearly not.

please help any advice please!!!!

Nicola8100, January 2012

The second thermostat is a frost stat, make sure it is set to 0 degrees, it is most likely set to about 10 degrees. The frost stat will overide the boiler what ever it is set on as it is designed as a safety device.

bill, January 2012