my 60 plate vw passat 2.0 diesel turbo is having clutch issues

when drviving at 30 mph the clutch is fine but when i accelerate to 50+mph the clutch pedal dops to the floor leaving about 20% of room to move the pedal to change gear then when i return to 30mph the medal goes back to normal after a few seconds

tonight though the pedal is staying down.vw reckon its mster or slave cylinder but want £90 to diagnose the £300+ to repair