I find the spark electrode lead gets worn as it passes throught the case of the boiler and arcs there rather than at the pilot assemby
unfortunatly you will need to remove the case by undoing both the butterfly and small nut just above where the boiler stat is and then pull the bottom of the case towards you and then lift it up of the top of the boiler to check it and that is a CORGI job
but if you can see the spark inside the boiler then it isnt this, its probably the gas valve and that REALLY is a corgi jobbie
P.c.b. , not sensing pilot flame .Flame on pilot could be too low , poor earth connection can also give this symptom . Is the polarity to boiler correct ? Normally , the fault as you describe can be rectified , by renewal of the p.c.b. I would recommend you employ the service of a c.o.r.g.i. engineer , with experience of this model .
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