I beleive there could be a number of things that are causing you the problem of a coninually sparking ignition system such as : (1) When was the heater last serviced and cleaned out, (2) The ignition switch connection could in itself be faulty by failing to allow a passage to earth for the spark lighting circuit, (3) A poor electrical contact or earth on the flame proving circuit would also create the same fault. I am not trying to be evasive here it is just that there are a number of things that could cause this annoying problem, my advice therefor is to contavt your local Pyrox agent and get them to check it out for you and service the heater while they are there to minimise a repeat of the same problem.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians