Okay; so here's a way to get a decent amount of pull on the element (assuming it is just the element that is stuck).
Start as before (post 4), but drill all the way through the element with your spoke-drill. Thread some flexible steel cable (more bike technology) through the element and recover the end through the injector hole with a piece of wire with a hook on the end. Tie a knot in the cable (a foot from the end) and pull it really tight. Pull the knot up against the element, and keep pulling. If it won't move, withdraw the cable, cut the tapered end of the element of a complete glow plug off, and tap it in until it pushes the stuck element a few mm into the head (be careful not to damage the seat). Rethread the cable and try again. When the knot jambs against the head, swop to a thinner cable, so it's knot will pull through the hole. EGR cleaner will probably help.
If you end up with the element sitting on the piston, I recon Alexie's flexible pipe and vacuum is your best chance (the element is only weakly magnetic). It can also be used to vacuum the debris off the piston.
We don't know why the plug broke. It might not be held that tightly, and failed due to corrosion/poor manufacturing etc. If you leave it there it might fall into the cylinder while the engine is running.
Edit. Just been for a walk and had another idea.
Thread the bike cable through the drilled element and out of the injector hole, solder a small lump of solder onto the cable on the plug side. Cut the cable behind this and pull the cable through until the solder enters the element and jambs in it. Tap the element into the cylinder and pull it out through the injector hole with the cable.
Last edited by R.T.; 13-11-15 at 15:58.