Baxi solo3pfl 50 radiators heating when system turned off?

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!!!!