Jools
Had a similar issue on my 996 a short while ago...
first of all the slave seal went - sorted that out with a standard kit.
I also had judder off a nearly new lightweight clutch/plates setup. (even after numerous cleanouts and an open cover).
after a few calls with JHP - it was suggested fitting 2 dished plates to the clutch pack, mine at the time were all flat and the stack height was spot on.
Fitted both dished plates in and ended up with no clutch (impossible to get neutral) - so have now just got one dished plate in and I can just get it in neutral (sometimes).
I did consider also fitting a larger slave but at the time heard I would loose travel on the clutch so avoided it.
...reading this back it probably won't help you at all, might help someone else out though.
regards
les