My Baxi Solo's main burner wont fire up. Why?

My wall-mounted Baxi Solo 50/4 PF didn't start up yesterday morning. Fan was running but no pilot and it still wouldn't light after pressing the reset button on the side of the control box.
Had a go at cleaning the pilot, the combustion box and control box, checking inside control box for anything obvious and blowing all the muck out.

On turning the boiler back on, the pilot now lights (all 3 indicator lights come on) but the main burner is not lighting. I suspect something in the control box may have gone but before I buy a replacement does anyone have any other suggestions? Thanks.