Worcester Bosch Greenstar 24i Junior boiler - No heating?

We have a Worcester Bosch Greenstar 24i Junior boiler and our thermostat is a Drayton Digistat RF1. We have hot water, but cannot turn the heating on either with the thermostat or the timer on the boiler. The Wireless controller is sending a signal to the reciever but this wont turn the heating on, neither will the override button. Any idea what the problem is and can we solve it ourselves? Thanks.