I believe I had a similar type of problem with the same boiler type but mine would not light without continually resetting the switch over and over again. The process would commence on either cycle but the boiled would trip out and the red light would come on. I tried all the usual suggestions (no small expense !!) until I finally decided that I seriously needed to understand how the thing works. What I discovered was that the fan, although it was working, it wasn't rotating fast enough when cold to increase the air pressure within the chamber, this acts like a sealed unit with the steel cover in place, and trips the air pressure switch (top right) which turns on the gas and lights the boiler. I found I could demonstrate this by removing the rubber tube from the flue ring (inside top centre), turning on the boiler, blowing gently into the tube and the boiler would light every time, as soon as you stop blowing the flame ignition stops. The cure was simple and cheap, I got my trusty tin of 3-in-one oil from the garage, turned off the power to the boiler and oiled the shaft of the fan in-situ, I found that a bendy straw fitted snuggly over the spout of the oil can and gave me good access to the fan shaft, do not flood it though, a little oil goes along way. I then ran the boiler without lighting it to let the oil do its work. Fingers crossed, the fan now whizzes round and the boiler lights when it should. I can only conclude that initially, by continually resetting the boiler the fan shaft must have warmed up sufficiently to spin fast enough to operate the pressure switch. Might sound crazy, but it worked for me and the tin of oil is cheaper than all those uneccessary replacement parts I ended up buying. Hope this helps.
David from Watford