I offered the suggestion of the blind rivet tool not so much to imply that this is the correct option but more to provoke other ideas that will allow some method of extrating the remaining glow plug stub.
Other possible options are the application of heat via an induction tool...
... or maybe this small bolt extractor can get a grip on the remainder of the plug and rotate/pull it out...? This is the same idea as the blind rivet tool - it has a section to grip the element...
