hi i have the unfortunate luck to own a house with the celsius 25 and have had experience with others on our estate.
the issue at the moment seems to be that it will not fire, it goes through the start up sequence but after the 4th attempt locks out, i thought aha thermo couple but it's electronic ignition, so phoned Keston direct and they said spark electrode, so after an almighty muck up on the behalf of HRP, i finally have one today a week later i've just fitted it and hey presto still doesn't fire, my meager conclusion draws me to gas valve as there is gas before the solenoid but nothing after or the pcb board as i haven't heard the thing spark (even tried it like you do with a car external with an earth point secured to it) help cause it's cold