I've found that the spark plugs that can be used are NGK CR8E or NGK CR8EIX. Just need to figure out how to get the plug out, cos' it's buried deep in the engine head. Almost as if the engine was built around the plug. So again, which size spark plug socket wrench? Thanks The spark plug socket wrench that I use for my auto works... tried it earlier, but hadn't seated it properly I guess.
I have resorted to using the one that came with the bikes tool kit as the walls of my other spark plug sockets were too thick to fit in.