Help me diagnose my clutch problem please.

This morning, the gears crunched when I changed into 4th. From that point onwards, I could not change gear without rev matching. I also can't change gear when stationary unless I kill the engine. Pressing the clutch does actually disengage the gears as I can set off and stop using the clutch when I'm in a gear (that's the weird bit).

I've checked the clutch fluid level and it's full.

Clutch pedal feels normal. Bite point has always been very high - dip it a couple of mills and the clutch slips. I changed the clutch fork, salve and flexi about 6-9 months ago.

Here are my thoughts in order of likelihood:

1. Air in system.
2. Master on it's way out.
3. Clutch worn.
4. Gearbox fault (something like adjustment).

You know the answer already, whilst you are in there I would also replace the slave and the master then its all done.

Actually, I forgot, I replaced both the Master and Slave together with Delphi ones not long ago - not just the slave, so they should be fine. Although the level was pretty full, there was a vacuum present when I opened it. I suspect one of the unions must be a little loose, I'll have a nose around and see if I can find a leak but It must be very slight.