Worcester Greenstar Junior 24i intermittently firing up and turning off?

I have a 2.5 year old Worcester Bosch Greenstar Junior 24i boiler. When I turn on the hot water or central heating the boiler attempts to fire up and then switches off after 5-10 seconds and the reset and service light comes on. It then attempts to fire up again and again the reset & service light comes on. After the 4th or 5th attempt the flame will light up and stay on consistently finally giving me hot water!

I have had the boiler serviced and the engineer couldn't diagnose the fault but thought it may be the air pressure switch. Another engineer had a look at it but didn't know what the problem was either and said I should get the PCB and flow sensor changed. I'm a bit worried as nobody seems to know what the fault is and I don't want to end up forking out a huge amount of cash for parts I may not even need.

Could anyone advise on this please?