How can I fix this Valliant problem?

I have a Drayton Wireless Digistat which says the heating is on but the boiler is not kicking in - wireless receiver shows green light and signal being received.
Boiler is Valliant EcoTec Plus and hot water is fine.
What is wrong? I have changed the batteries in the Digistat but still no luck.

Gillian
January 2010

dhw flow switch is probably sticking

Ian
December 2009