Constant receive alarm on Drayton digistat SCR?

I have a Vaillant Ecotec pro combi-boiler (recently serviced) a thermostat in my hall and a Drayton digistat SCR receiver next to the boiler. I don't have a remote which a lot of people seem to be commenting on, but have had no problems up until recently. The receiver now just has a red led next to 'receive alarm' and the heating wont kick in until I hit the override (however this just keeps the heating on and doesn't stop at the temperature specified on the other thermostat). It works only sometimes, but leaving the whole system off overnight doesn't seem to sort anything. Is there an easy (ie nearly idiot proff) way to solve this?

Will
February 2011