How do I fix my Potterton Netaheat MkIIF 16-22?

My boiler starts it's cycle on room stat demand. The internal fan runs, the pilot ignites, after 20 secs the main burner ignites and all is well. The pump appears to work (a)screwdriver to ear and against body of pump b)spindle rotating behind the vent cover) Down stairs rads get slightly warm and then, after about four minutes the main burner shuts down and the pilot remains on. System checked and vented, all seems well. I have replaced the Mercury vapour switch, the pilot ignition system and the 11 pin relay switch with no effect. Having waited eight days for next day delivery, Priceright Heating now tell me the PCB for this boiler is obsolete. Any help would be appreciated