I am having the exact problem as original post. I fixed mine by firing it up with the gas isolator closed. This went through a purge/start up cycle and did not light. I then turned the boiler off, switched gas isolator back on, switched boiler to HW and heating (60) and it stayed on. ! This must have cleared something in the gas supply side I assume. This was after 2 hrs faffing and constant red lockouts.
It seems that lock-outs can occur for a variety of reasons (many stated in the post previously). In additrion to voltage/gas supply etc. also note that the blades of the small 3 " diameter fan that cools the PCB need to be kept clean as if it spins slowly it is detected and a lock-out (red light) can occur.
Also - finally - if your boiler has an electrical heating element then the physical overheat trip may have operated - this is accessed via a small peephole and can be re-set by inserting a 3 inch long probe and pressing gently.
However I would be very interested in the procedure you state above Vanity, please share. Thank you.
It would be great if Moonshadow could share the final fix too.