Remeha Avanta Plus not not activating heating?

I have just moved in to a new property with an Avanta Plus Combi Boiler. It works fine for heating the Hot Water. However, the central heating function works, but only when operated manually. For example, when you turn it on using the thermostat, it turns on but does not automatically switch off. When you manually turn the thermostat down, it switches off ... But then won't come back on automatically as the temprature drops! All very frustrating. Any help greatly appreciated.