Please help me diagnose my clutch problem

Hi,

I have a 2006 1.5dci Megane. The other day I noticed the clutch pedal was lower than usual, it was only half way up. I was on my way to the hospital to visit my son at the time so I had to keep going. I parked the megane in the multistory car park at the hospital and when I returned to the car a few hour later, I couldn't get it into gear. The clutch wouldn't disengage. I checked the reservoir, it was full of fluid. No sign of any leaks.

Eventually someone told me to lift up the clutch pedal with my foot and pump it. This fixed it, temporarily perhaps. I drove home, this was a 40 mile journey. The clutch felt normal. I haven't really driven it since but I drove it around the yard today and the pedal still felt normal and the clutch still worked fine.

I suspect either the master or slave cylinder is failing but I don't know how to tell which it could be. My haynes manual says the slave cylinder is inside the bellhousing with the clutch plates so this would probably be very expensive to repair.

Can you tell me what you think this is please?