he wanted to personally phone baxi customer support and complain!!!!! ]
good luck with that! LOL
Seriously, there is a limited number of reasons.
Dirty / Poor pilot burner - seems unlikely, going on your previous posts
Faulty mag valve in Gas valve - possible but unlucky
Faulty thermocouple. Even new this is the most likely, and for the minimal cost risk, worth a punt. I always use genuine honeywell tc's not the carp universals
Flame disturbance / oxygen starvation. Actually possible, but if the fitter has tested flue flow and spillage, it's unlikely. Assuming main burner doesn't come on withn a bang.
Pressure fluctuations: Possible, I suppose, but I'm not convinced. If you phone Transco, or whatever, they are now called, and tell them your fitter has reported occasional low pressure, they will come out. They have to, and will test pressure, but your fitter could do this anyway, and should have tested burner pressure when he last attended. Be prepared, though, for them to cut you off!
If Scooby lives local, take him up on his offer of an old valve. Your fitter will probably be happy to swap out, I would in his position. Good luck.