Get a stop watch. Grasp the gas valve control knob and turn the knob to off position to extinguish pilot and start the stop watch at the same time. You should record over 20 seconds on the stop watch to hear a click from the gas valve. Send me an email on Rannochservices@ntlworld.com if youwant further insight into how this boiler works. When you say you have checked the fan and flue system, have you checked the entire length of the inner tube? Have you checked that the inner tube slots into the terminal such that products of combustion are not re-entring the outer tube? Anyway, if you want answers you have my email.