It was really wedged in there
[Linked Image from i3.photobucket.com]

Eventually after some time I got it out
[Linked Image from i3.photobucket.com]

I used some rolled wet and dry to clean the two surfaces
[Linked Image from i3.photobucket.com]

[Linked Image from i3.photobucket.com]

After cleaning both surfaces I put them in the vice and swiveled them back and forth for a while to ensure the moved freely.
[Linked Image from i3.photobucket.com]

Upon reassembly I noticed that there was a sleeve/crush washer missing from the clutch housing cover so I made one up from a piece of dynabolt tube.
[Linked Image from i3.photobucket.com]

Fitted perfectly and allowed me to torque the bolts up evenly.
[Linked Image from i3.photobucket.com]

Next time I'm at Stihl I'll try and source an original.

Ran the machine after getting it back together and it ran like a charm. I guess sitting around and some moisture can get in there and sieze things up.