hi,once you removed the wires and the forked bracket, spray a little lubricant around the glow plug,get a reasonable pair of grips and just turn back and forth, its not screwed in. just sits in cavity. the glowplug bottom,only goes mm in to the haed so you'll have more chance with grips or indeed get a angle grinder and gently make a new slots for a spanner.
hope this helps Rob