You can buy a set of screw extractors, should not be more than L10 in a toolshop. Then you drill a hole approx 1/2 of the diameter of the bolt, well centered, straight down the bolt.
You should punch the bolt's center before to drill, and the hole does not need to be more than 15-20mm deep if you have an extractor of the correct size.
You need a good drill bit, as these bolts are of good strong healthy grade of steel (or they should, anyway!).
Drill very straight and very centered. Once drilled, insert the extractor, and unscrew either with the appropriate square-holed handle, either with a suitable flat spanner.
Use abundant penetrating oil before to unscrew...
Enjoy!
Philip