the heating on my valliant vcw 242e combi boiler will fire up?

the central heating on my Valliant VCW GB 242E combi boiler does not fire up when the heating is switched on, There is 1 bar of water pressure and the hot water seems fine,The flue fan runs and the rads have been bled. The heat exchanger has been replaced in the last 2 months and the heating has worked since then Please help.