The best way to shift these is to buy a can of dedicated releasing fluid (not the stuff in the blue and yellow can) and fill the glow plug recesses with the fluid after stopping a hot engine. Do this several times.

After a few sessions, soak the recess and with the engine still hot, hammer a Torx bit with a solid tip into the recess, selecting one which is a good fit. If it's too tight, it will expand the plug body, making removal more difficult. You may have to dig out some of the magnesia filling to gain good access.

Take a 3/8" drive extension and fit it with a "T" handle - not a ratchet handle. The idea is to apply pure torque with no bending moment. Apply force carefully, tightening and slackening alternately until you get some movement. Be prepared to add more fluid and to take some time over this - but the plugs usually come out unless they have previously had the gorilla tratment.

My recommendation for fitting new plugs (which is not in accordance with some others) is to fit Beru plugs only (stainless body) and to apply copper based anti-seize compound sparingly to the thread only - not the taper seat. Torque to about 80% of Beru's recommended figure - which is helpfully printed on the box. Glow plugs do not need to be installed very tightly and distorting the hollow plug body will cause all kinds of problems.

Over many years of glow plug fitting and some work in glow plug development, I have found this method works best. Keep the taper seat free of anti-seize.

659.

I was faced with changing glowplugs on a 1.3 Corsa and did some research before commencing and sure enough these suffer the same problem during removal. They are very long pencil thin and any serious twist will just rip them in two.

I decided not to proceed because a cylinder head removal for a car worth £700 was not what I wanted. So it was off to we-buy-any-wreck and that was that.

I am very interested in the above solution from 659FBE though. Being a bloke who gets his hands dirty I like advice such as this. I shall put this one in the vault for later. Nice explanation, thanks for sharing.

I always use a torque wrench to undo glowplugs, Beru list maximum torque for the different types of glowplugs. I set the torque wrench to that and if they won't undo at that torque I do as 659 said and give them a good soaking for a week. I used a special ceramic grease on the threads when fitting the new ones.