Hi Chris. I now have some more information for you. I have a little knowledge of marine engines as I used to build high performance race engines but that was way back in 1980 before EFI was about. It was all carburators in those days. The cylinder not firing is No1 ( the one closest to the front of the ski). I have taken the plugs back out and turned it over with the plugs resting on the cylinder head. Both plugs are sparking good health sparks. The No 1 plug showed no signs of use as both plugs are brand new but it was clear to see that the No 2 plug had been working. I think that this may point to a fuel issue. There is over half a tank of fuel on board so I'm now lost.