Central heating not working boss therm BP524RF?

Hi common problem, boiler central heating not coming on after winter. Tried the instructions given in other postings(thank you), but no central heating, water ok.
green light on receiver all the time.
green and orange lights when the central heating should be on.
orange light flashes in between on-off, off-on change over.
no instruction manual left.
The system is just over 3years old and this is the second thermostat and receiver I have had!
Any thoughts