Well hopefully this is now resolved!
Last weekend I had chance to swap over the suspected faulty injector with the one next to it to see if the problem moved. However before I actually swapped them we thought we'd try and fire the injector without it being in the cylinder head to see if the spray/nozle looked ok.
This is where we discovered that the actual injector nozle isn't sealed into the injector unless it's clamped into the cylinder head as fuel went everywhere when the injector tried to fire but came from the sides rather than the tip of the nozle.
We then tried to put the injector back into the cylinder head and it wouldn't seal with fuel leaking out when the injector fired.
The cylinder head itself had loads of salt deposits on it as well so what we did was basically clean everything up. Cleaned all of the salt and crud off the cylinder head until the injector slid into place easily.
Thinking the injector was probably knackered now anyway we threw caution to the wind and soaked the injector nozle (which we removed from the injector body when it wouldn't seal) overnight in redex injector cleaner. We also cleaned the injector body of any salt deposits and put some redex into the end of the injector where the nozle fits. We didn't completely strip the injector core out as we didn't have replacement o rings so figured we'd hope the rest was ok and try and clean the tip.
Two good sources of info on the injectors were these two web pages.
Ficht fuel Injector - Rusty Discharge?? - Page 4
After a 24 hour soak we re-assembled the injector, bolted it back into the head and fired her up. She ticked over a dream with no missfire at all, revved to around 1500rpm and still fine.
So yesterday we took her out for a test with some redex in the fuel just to give all the injectors a bit of a clean on a local water skiing lake, she ran no problems or missfires for around an hour so hopefully we've cured it.
Many thanks to everyone who's given advice on here and also Ian at Liverpool powerboats and Mike + Paul at South coast Marine who have all helped find the problem.
I'm hoping we can make the next ribnet cruise in our own boat!