Glowwork Ultimate 50 boiler - pilot keeps going out?

The pilot light keep going out but immediately relights itself so I am getting hot water and heating. I have had a Corgi engineer look at it but they are baffled. The boiler has had a new air pressure switch, pilot assembly and hose, gas valve (which was blocked), fan and PCB board but to no avail, the problem persists. The gas pressure has also been checked and is fine and apparently it is calling for heat so my engineer has reached the end of the road as he says there is nothing left to replace on the boiler. What could it be?