Is your Allen key still in good shape ? or has this occoured because the corners have worn from the Allen key?

Without seeing the state it's in its not easy to suggest a fix as the fix depends on the state and what you have available to sort it.

Suggestions I would throw in are;

1) Before starting, make sure the hex bolt is clean, properly clean with no debris inside. Then try a new allen key (with nice clean corners on it), you might just have enough contact to remove it. I'd be inclined to leave it (bolt) soak for a while with a good quality release oil, before starting. The stuff Scaffolders use to service the fittings is very good for releasing siezed bolts.

2) Mole grips, if you can get good purchase on the offending bolt, if not I wouldn't even try it as you could make things worse.

3) Heat. Getting lots of heat into the bolt will help, but you need to be able to grip it first!

4) Screw Extractors/Easy Outs. As long as you can get a good grip on the tool, in a straight line, otherwise they tend to snap.

5) using an oversized Allen key, hammer it into the bolt, or if you have a Torx bit, they work too. Using an impact driver will help.

Just a few, I'm sure I'll think of more as the day goes on!

Good luck

__________________
>-------<

Never underestimate the power of stupid people in large groups.