How do I get this glow plug out?

Just tried to replace the glow plugs on my 205D and the second from
the alternator side is broke. Don’t know whats happened to it but the
12mm bolt bit has somehow detached itself from the main body of the
plug.
The bolt also doesn’t seem to have a thread anymore and there isn’t
space to get an adjustable spanner on the plug.

How do I get this out?

 

Sounds like a job for your local engineering shop,
but you might need to remove the cylinder head to gain access to the broken
plug,
don't be tempted to knock in some kind of easy-out tool,
this could cause the plug tip to fall off and damage the piston,

good luck
Fitzy

 

Unless you're especially fond of the car I'd just replace the other 3
and carry on driving.