VZ800 Clutch started slipping- help

Hi all. So picked up a Marauder last week 20,000 miles on the clock. First ride last week faultless. took her out again just now and the clutch is slipping in high gear? Clutch is letting go right on the end of the lever play as well.
So first is there any reason apart from a worn out clutch that this might suddenly happen? I have done nothing to the bike at all.
secondly what do I need to replace the clutch?
do I need top replace friction and steel plates and springs? 
Also will I need a new gasket?
Do I need the special tool see on ebay for £25?
anything else?
thanks
P

Hello, its a bit contravertial but if someone has put mineral oil (car oil) in it then I beleive this can make the clutch slip, it may be worth changing the oil & filter, use semi synthetic bike oil, a good one is 'Silkolene'.

Mine slipped when I used full synthetic & it took a while to get it back out of the clutch plates. Only use natural oil that's what it's designed for. Have to take it easy so you don't glaze the plates. In the end I installed a set of the extra heavy springs. Now that I'm 60 & don't ride around at full throttle anymore I wish I knew where the original springs are.

Stripped down, everything measured within tolerances but the plates were blue. Replaced everything put in new tighter springs. Changed the oil to semi synthetic, new gasket. put back together - sweet as a nut now and no leaks! Hurrah. Been out for a blast and what a difference. Now to get the dent out of the tank and clean up and/or replace some chrome.