Allen bolt out of head - help!

Just painted my R1 based bikini fairing and tail and fitted a set of drag bike bars when I noticed the rhs of the rear cylinder one of the allen bolts that go up through the barrel into the head had dropped down so that the head of the bolt was resting on the casing.
After much swearing I managed to get it to go back up and have managed to screw it back in using my fingers.
Obviously I need to tighten it back up but there is very little space.
I thought I'd try cutting an Allen key back so that I have enough space to get it to engage with the bolt and stand a small chance of being able to tighten one flat at a time.

Does anyone know what size allen key fits this bolt?
Anyone have any ideas?

Thanks

Done!
I went down to my local hardware store and bought the cheapest allen keys I could find.
Heated the bend until it glowed dull red and then bent it to a 45deg angle (much more acute).
Used the angle grinder to cut and reshape the piece so the short end was about 12mm long and the long end about 22mm long.

Took an hour to turn the bolt through 2 and a half turns and another hour to torque it up.

good effort! now get us some pics of this bike!!

Glad you got it sorted mate, sounded like abit of a worrying problem to me!

What size allen head was it - just for future reference!?