my worcester bosch 240?

lately when the heating is on but has cut out because the rooms/combi is up to the correct temp.when it tries to cut back in all you can hear is the ignition continuiously clicking but no gas ignition.if you turn on the hot water tap the pressure switch clicks but still no ignition.all of a sudden it will fire up and work.Whats going on.also there are times when heating is off and turning on the hot water tap gives same results.HELP.