Clio 1.5DCi 2001

My sons clio 1.5 Dci was a complete tuck up job when we bought it. The list of faults was endless. After a week or so, the clutch started slipping so I ordered a new 3 piece clutch kit which I fitted clutch slip sorted. About 8 months later, he started complaining that the clutch biting position was varying; sometimes biting very close to the floor and gear changing was getting hard. I fitted a new cable and he stopped moaning so I thought that was it. Come winter, he complained that the car wouldn't go into gear when cold, I told him to start the car in gear with clutch depressed, and again, he seemed OK with this. Wind on another 6 months and now the car is very hard to select any gear. I took the car for a drive, first off, I put it into first and started the engine, the car tried to jolt forward before the clutch released and changing into third could be very tricky, taking a few tries to get it in with a clunk. My first thought was gearbox mount, as the stick seemed to be jerking a lot, I replaced the top mount as that was the only original mount left I hadn't changed already. No change. You can just about change gear with double d' clutching. It doesn't want to engage gear at all from rest but once it has selected a gear at rest, the change seems to be all smooth. My suspicion is that the diaphragm spring has somehow failed, you can see the lever compressing the diaphragm normally so it's not cable or lever, I can only imagine that for some reason, the diaphragm spring isn't releasing the pressure plate.

What I don't want to do is fit another new clutch only to find that the gearbox synchro rings are knacked or something.
Am I missing something obvious here?