>>I can see that the tip seems fine but the body of the GP that goes into the larger part of the bore has some carbon in it.

This is exactly as per my experience. When I did mine, 4 of the glow plugs came out really easily, just a case of cracking them loose, and unscrewing by hand.

The two which were (really!) difficult had leaked carbon beyond the sealing taper, and the carbon was sticking to the main body of the glow plug.

I think that using an anti-sieze upon reassembly is important, but ensuring a good seal on this taper is IMO vital. To this end, I think that putting the glow plug in at a reduced torque (in a mis-guided attempt to make it easier to remove next ime) might actually be counter-productive, because the reduced end-load will compromise the taper seal.