only just found this about a snapped bolt.
In the past, i have tapped the end of the stuck bolt with a hammer and gentle force to try and jar the threads in block before using mole grips or welding a nut on stud. the heat from welding helps out, even with the "shocking" treatment, and if the weld is good, you get better grip than stud removers.
Must admit I never tried this on my disco, but done mobile car/van repairs once upon a time, so had plenty of practice with "problems"