How can I fix my Potterton Promax Combi HE 33 Plus A Range Boiler?

Hi

We have a Potterton Promax Combi HE 33 Plus A Range condensing combination boiler fitted along with a Myson MPRT programmable room thermostat.

The boiler/heating is continuing to run even with the thermostat/heating switched to off. However, the hall radiator is the only one which is hot, all the others are thermostatically controlled and are stone cold.

I have switched the boiler to hot water only and the boiler has now shut down on the heating side as per normal operation. Any ideas what the problem might be?