Thanks for your reply Max but apologies - I didn't word my question properly.
"Seemed that the plug's insulation had broken down."
What I meant was the insulation had stopped insulating. There was no sign of any of the ceramic insulation material being damaged, but obviously there was high voltage going through the insulation material. The material around the central electrode exposed to combustion looked 100 % intact with normal deposits after years of use and I cleaned that carefully. The material outside the cylinder head also looked perfect, no cracks, clean and shiny.
When testing the plug by putting the body on bare metal there was no spark across the electrode gap and when testing by separating the plug body a few mm away from the metal earth there was a very obvious spark jumping across to earth.
