Glow Worm 35f Fuel saver won't light main burn?

I have an old glow worm 35f fuel saver, positive pressure boiler. The pilot lights intermittently but won’t fire the main burner. When the pilot lights it wavers then goes out again, the unit has a healthy spark. I have replaced the following components so far
• PCB
• Both solenoid valves
• Overheat cut out device
• Selector unit/dial on the front

And I have checked the following
• Fan running, WD40’d only 2 years old
• Pressure switch, functioning ( Suction on negative pressure enables boiler start)
• Gave the igniter and flame sensor a little clean with a soft brush

I would really appreciate any positive help on this boiler, great site, keep up the good work.