Central heating confusion?

I seem to only get hot radiators while my boiler heats my hot water but as soon as that get to temprature even though my thermostat is calling for heat my radiators go cold I have a potterton f50 boiler. I have replaced the programmer (timer) made no differance. I dont think its the diverter valve as this appears to be working as normal while the water is heating up my radiators work as well but when the hot water is off and i try to turn on the central heating only i get no ignition on the boiler. (programmer shows its on) Please can someone help this is so frustrating.