This happened to me a couple of weeks ago!!!
Check the fluid level in clutch reservoir and then check for any obvious leaks. If nothing follow the solid pipe from clutch master cyl. and you will eventually find a flexi-pipe (passenger side). Now with either a pair of mole-grips or even beter a brake pipe clamp, clamp this flexi-pipe then go and pump the clutch pedal several times if the pedal goes solid, the slave is at fault, if there is no difference then its the master cyl.
When removing the slave be wary of the pushrod between clutch fork and slave cyl. as it is easy for it to drop inside the bell housing and it detaches from the cluth fork.
Goodluck!