Glow worm ultimate boiler not lighting.

My friend has a Glow worm Ultimate boiler that is just clicking and not lighting. He can hear the fan running and can feel a draught outside at the flue.
Sometimes the biler will light and perform correctly until it switches off again and then the clicking restarts when it tries to light.
He has cleaned the pilot and checked that the spark electrode has not moved out of position.
He is competent enough to investigate the fault with a guide as to where to look.
Any suggestions would be gratefully received

He may be competent but he ain't legal.

This is almost certainly a fault in the 'gas train' inside the boiler (if the gas is actually switched on!). To fix it legally requires a CORGI ticket.

if he deems himself competent he CAN work on his boiler or anyone else's boiler as long as he does not do it as a profesion or get paid, it is morally wrong but thats what the gas regs say. you have to be corgi registered and have relevant ACS if you work on any gas fittings or appliances connected to a gas supply. Back to your fault, how did he clean pilot? did he strip it down? if its sparking and not lighting, it will only be a blockage in injector, pilot solenoid or no gas.

Glad someone knows the law and regulations. Yes of couse the key is 'competent'
Thaks for the guide DJBunny. We got the circuit diagram off the GlowWorm site to help us.