Rounded Allen key head

Shock removed/no load on on it.

Yep. Shock has been removed, so load free!

If it's was me I would have got Dremel, cut a groove so you can get a decent sized screwdriver in it, then use a screwdriver what takes 'bits' you know the ones what slot in the end of a driver. Use the driver to put pressure on and get spanner on the driver end tip to undo the bolt. Before you try to undo put a screwdriver bit or old screwdriver in slot you have just made, then give it a few sharp, hard blows with hammer, this will shock threads and loosen any crap that's in there. If there's any places you can hit without damaging anything where the bolt goes in I would do that also. Failing that I would just go with drilling it out, using a drill on a low speed with the best hss bit you can get your hands on, going from smallest you can, working your way up to bigger bits until all that left is a tiny bit of metal before threads. Then as has previously been mentioned use tap or same size bolt to drift the old bits of thread out. If you don't have a tap to do it then ask on here, I may have one depending what size you'll need.

Leigh

Re:

Thanks Leigh. Will be attempting this at the weekend though don't have a dremel to use. Did look for a budget version in B&Q but no banana. Will post the results!

Before you start working your way up use the biggest drill bit that will fit inside the hexagonal hole and give it a quick push. This will create a little spot bang in the centre to then begin stepping up from the smallest. much easier the centre the hole this way.