Glow Worm Space Saver 60R CF Mk2 turns off after 3 mins?

The pilot seems to be strong and the flame is on the Thermocouple, I have checked for overheating, of which there is no sign, the water is pumping around the boiler, I have linked out the controller to eliminate a possible broken or faulty cable. I have removed the phial from the pocket, to try to run the boiler without the thermostat operating, the gas pressure would seem to be Ok, on other appliances, but the burner shuts down and will not restart for approx. 20 mins. Any ideas ?

Does anyone know where I can download a manual.