Did it today - the 998 has got one big nut held in place by a bent over lock washer. Its impossible (or I didnt work out how to to do it) to remove the front sprocket without removing the clutch slave cylinder. Three bolts and that comes off easy enough. There is an o ring on the back of the housing that you have to be careful when reassembly, but it just comes off and goes on again, no rebuild or anything needed.
Put it all back together again but haven't had time for a play today, if not tomorrow definately on the way to work on Monday
cheers,
Ben