I've been doing some reading up on this particular boiler, Opto.
It's... how can I put this........ not terribly well regarded in the world of boilers :o(
There are at least half a dozen possible reasons for this intermittent fault. Even paint flaking off the inside of the flue pipe has been known to stop the burner igniting!
A blocked condensate pipe is apparently quite common.....
Have a look here to see if you can find it...........
http://idealheating.com/downloads/manuals/203318_inst.pdf
The other possibilities can only be dealt with by a skilled boiler guy. All these guys have different areas of expertise. I suggest you ask Ideal for the name of a local guy who knows these boilers from experience.