Ben,

Did your sprocket have the holes for the two retaining bolts, and a splined locator section as in 916/996 set up, or was it different to this ( i.e. 998 specific) I have had the sprocket for 1 year and haven't fitted it yet!

Sorry about the slow reply, couldnt get on yesterday.
My sprocket was 998 specific as it didnt have the two locating holes. It was supplied by Tim at Cornerspeed. I just slid the it onto the spindle (after a bit of rotation to find the correct position), slid the retaining washer back over it, did the nut up and bent the washer back over the nut, just like the OE fitting.
Hasn't fallen apart so far!! :D

Ben