Stuck glow plugs HELP NEEDED PLEASE

I have a 10 year old 7540 that has the later engine that has glow plugs rather than a thermostart. All the glowplugs are now open circuit so it does not start in cold weather.
My problem is that I can not get the plugs out! I have squirted WD40 down the side of them and left it to soak, but am worried that I may break them.
It is very hard to get a ring spanner in and looks like I'll have to remove the injectors or at least return pipes (that look cracked and could do with renewing anyway).
ANY HELP OR IDEAS GREATLY NEEDED.

Remove the injector return pipes & use a deep 6 sided socket on the glow plugs

you must do as Alzet says but bee careful..... two of the plugs on my 9641 broke of when i tried to remove them..... but i got in two new ones and it starts good with only two glow plugs.....if they break you would have to take the cylinder head off to gett them out 69) so be careful thats just a little tip good luck

you must do as Alzet says but bee careful..... two of the plugs on my 9641 broke of when i tried to remove them..... but i got in two new ones and it starts good with only two glow plugs.....if they break you would have to take the cylinder head off to gett them out 69) so be careful thats just a little tip good luck

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That's whats worrying me! The idea that they snap off and leave a hole into the cylinder. I'll get a long socket try to start the beast to get it hot then give it a go. Bit of a pain the the botty. My old 1987 62 hp zetor had no cold start and was the best starter I've ever had out of about 20 diesels.
I unwired them yesterday and they are all dead.