Come across this many times Clive, if you have access to welding bottles or a good heat source and enough thread to get two nuts on and lock them together, use the first nut to undo it.
Best to get it red hot a few times then screw the nuts on and then unscrew the stud, never failed me on many years of studs shearing.
If you don't have a good heat source, do you have a local garage who will have the bottles and for a few quid would do this for you, maybe safer if using gas bottles for them to do it.
Don't force it if still tight, reheat and gentle force will move it