Thanks for all your suggestions.
I didn't consider a torx socket. My local tool shop supplies them in individual sizes. Seeing as I rounded the 8mm bolt (and the allen key was undamaged) maybe a 9mm torx is the way to go?
So, I have a plan of attack:
1. Try hammering a torx socket into the rounded bolt and give it some welly with the socket spanner
2. Break out the drill and get a bolt extractor on the go
3. Find a friendly welder to mig weld an allen key to the bolt
4. Break out the saw (and the credit card).
The thing is, I'm diligent about greasing bolts. I suppose the logical answer is I overtightened it in the first place. Hmmm, you live and learn.