It's a 10mm allen head bolt and it sounds like its rounded off. The bolt itself is well know for being difficult - It's unusuall for people to have to remove the strut and the fork as one unit, cut both ends of the bolt off and then hammer it out of the fork using lots of harsh language.
Either way it sounds like you're going to need a new bolt before you go any further.
It's not unusual for "novices" to break the fork, during the removal process, too if it's the alloy type fitted to later 156's and all 147's...
We use 12 point spline socket on rounded off allen bolts - its relativly effective but you have to hammer it into the socket, ruining the bolt. The other option is to use and irwin easy out on the outside circumference of the head.
I'd also recommend that you use copious amounts of copper greese when you refit the bolt.