Worcester Bosch 24i Junior no spark ?

I have the red flashing light, once per second. Went through manual fault finding. I replaced the spark electrode + harness, flame sensor + harness, PCB, APS and the gas valve, still to no avail. The fan and the pump kicks in and then when it requests a spark it just cuts out. I've had a qualified engineer look at it and he keeps going back to the PCB, but it's new, so we are both completely puzzled, has anyone got any other suggestions besides a sledge hammer?