my perfecta 24 powers up extract fan motor runs but no ignition?

when boiler is switched on the boiler cycles you can here the fan motor start up but boiler fails to light (no ignition) have checked all the micro switches, the thermistor, ohms vary in hot and cold water, checked
the thermal cut out,checked ignition coil ohms value, checked ohms value on valve coils, changed the pcb, and selector switch, can a water pressure have a bearing as the highest BAR is just over 2.2bar can't get it up to 3bar.