worcester 24 cdi.no heating but ok on hot water

[fabio]

Was called to worcester 24 cdi combi boiler.boiler fires up ok on hot water but when the room stat is turned up the heating doesnt fire?if i was to remove the room stat link which 2 is it that i remove from under the boiler?i have already taken the 3 screws out and found the wiring.also i have been told that the relays in the pcb can stick?any advice would be greatly appriciated.thanks