worcester 28i rsf has no central heating?

my boiler has no central heating but hot water fine. have checked following:-pump unit, combustion fan,resistance on thermistor ch. side, control knob shaft. boiler will not fire up for ch function hot water will. now thinking integrated circuit on pcb gone again,how can i buy one, they plug in and whole board is £175.00 they are made by pactrol with serial no 423101/sw/ic1/v1.3 any help appreciated