My 01 Y 90 TD5 has developed a clutch problem by loosing pedal stopping the clutch from working sometimes having to switch the engine off to get the gears changed, I have changed the slave cylinder and no difference is it the master cylinder?

I take it you have check bleed and re bleed the system.

Have you check the adjuster on the Master Cylinder could be faulty adjustment.

I guess other option is clutch fork, but sounds more like pressure problem so I would have thought you should be loosing fluid somewhere.