how to fix my reno HE30C combi series johnson & starley ?

my boiler is all on. my hot water is fine. but when i flick the switch up to put central heating on its not turning it on i was told last time when this happend by powerminster to try the thermostat in my hall to get heating on now this worked for them but its not doing it for me im cold thankyou.my thermostat is a small white box with a round dial with in that in the middle it has a set button also it has a digital screen thats displaying a triangle shape with circuls above that then a set of flames next to that then 20.c then a .5