How can i fix my Potterton Performa 30 HE Boiler?

Boiler failing to fire up; showing fan & flame fault indication. Dismantled boiler and checked fan on 240v supply & ok.Checked fan venturi system & ok. Checked air pressure switch and not changing state. Bought & fitted a new one and boiler still not firing up. Found the pcb (5112380) faulty; bought & fitted a new one. Now the fan works but the burners still not igniting. Checked new pcb & found no 240v to open up gas valve & strike electrode. Replaced pcb & still the same. Rang Potterton tech support and they say i should have 240v on pin 8 which should switch on to pin 9 when air pressure switch operates. Well on both new pcb's i'm getting half the voltage ie.120v on pin 8 but when it switches on to pin 9 it disappears to 0v. I'm puzzled, can anyone help on this one please!!