Further to my post yesterday, I have now replaced the air pressure switch but I still cannot get the boiler to ignite.
The ignition sparks if I blow down the tube to the APS but soon goes out.
I have also checked the heat exchanger and this is very clean.

It's pointless and confusing to start another thread.

Is the fan going to high speed.

Is the flue blocked.

Is the pump working.

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Pump working, fan working. Can only get ignition when I blow on tube from Air Pressure Switch.
Not sure what to do next apart from disassemble flue and check that nozzles to Venturi are not blocked.
Boiler is not very old and the PCB and thermistors have already been changed in the last couple of years.

Yes, but does the fan run without blowing in the tube.

When you changed the APS did you connect the tubes in the right order.

Are the tubes split or damaged.

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Fan runs without blowing down tube and both tubes are in perfect condition.
When you switch on the boiler the start up sequence is as normal, the only thing that does not happen is the ignition spark and ignition. I am sure that I fitted the APS correctly but I will switch the tubes just to make sure.

Tubes to APS checked and are correct, and actually you cannot swap the tubes around due to the length of one of them. Removed fan assembly and checked venturi, both clean. Flue also free from muck and intake quite clear.
I am thinking that maybe there is a problem with the igntion pcb although I am loath to fork out on such an expensive part.

I am not really sure what to do next.

A boiler go through a sequence.

The pump has to run, then fan, if it gets that far everything is ok, so you are left with the ignition.

If you look in the fault finding section it will give a couple of options.

Electrode, lead, pcb.

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Electrode, lead and PCB all exchanged and still no ignition!

I have ordered a High Limit thermostat as this figures on the fault diagram a couple of times. Something is actively stopping ignition.

If this doesn't work, I am going to attack it with a club hammer.