Thanks for your last Adz. Here's a new one:

My frontmost glowplug has snapped just below the hex nut section. I'm working with 3 glowplugs now but sooner or later I'm going to have to fix this. I don't know whether it broke due to seizing and being over torqued, or being knocked by something.

Can you pull the central rod out and use an Eezi-out on the casing?

 

Sorry mate - I've no idea, I've been lucky and never broken either glow plugs or sparks on petrol cars.