Clutch goes, then comes back again?? :S

Hi all, my clutch in my 1990 Defender fitted with a 200 tdi has developed a strange fault. I was driving down the road, stopped at traffic lights, when they went green I put the clutch in and selected first but it wouldnt pull off, the clutch was stuck engaged so I removed the slave cylinder and freed the clutch and limped home. I bought a new slave cylinder thinking this was it and changed it and it drove fine, got a few miles down the road and it did it again! the pedal also went hard like the brake pedal. I left it all day and jumped in it again later on and it drove fine again! I then changed the master cylinder and carried on driving untill the same happened again so I give up now it cant be anything hydraulic (unless the new units are faulty) so what could it be? I dont want the job of taking the whole box out to find its something else! If you leave it a few hours it comes back completely normal and you can go for hours then suddenly it will go again, its now developed a noise too which I have videoed here:
Defender clutch noise? - YouTube

anyone got any ideas?? Ive got a feeling its a box out job but I dont really have the time or space to do it at the moment so I want to explore everything first!
Thanks!

Cant open youtube links at work...but im guessing your clutch is about to say bye bye

it may be adjusted to far on rod in pedal box it can stop fluid returning and ratchet clutch on

Have you changed flexi hose.

No flexi on mine just a solid pipe all the way, I dont think its the adjustment but will have a play. to be honest I'd be happy if the clutch failed at least I knew what was wrong! at the mo I'm facing the prospect of removing the gearbox and it being all fine!