sabre 35 he combi boiler ?

i.v still got a problem with ignition on my combi iv tried cleaning the tubes & venturi i,v tried a new differential switch still the same.i,v put the old one back on didn,t see any point paying £35 for somthing thats not solved the problem original problem was boiler not igniting green light flashing every4-5 sec as normal fan runs waterpressure 1.3 let air in via inspection panel casing and the boiler would light now it won,t seams to be a pretty common fault but no ones got any answers .wizzyh59@hotmail.com cold and dirty