Im not sure if this applies to the disco slave ,however, usually a slave has a threaded rod that thrusts the clutch fork. To give you an idea of what Im on about, the slave body is roughly cylindical with a bleed nipple and supply pipe at one end and at the other end is a short rod sticking out. this rod is usually threaded to allow adjustment of clutch travel. Travel should be about 1-2 inches off the floor (the clutch pedal) before the clutch begins to bite. It is possible that you have insufficient pedal travel resulting in a lack of clutch release. In other words, your clutch isnt disengaging. This would cause a symptom like you are having.
Solution, remove the bolts of the slave, without disconnecting supply line if possible, and turn the rod in the direction that extends it. try a 1/4 ich to start. Make sure you test your pedal travel afterwards using the 1 inch off the floor before it bites info. If you have too much travel before it bites, your clutch will never fully disengage and you will wear your clutch out fast.
worth a try. but it would seem to me one way orr the other your clutch isnt fully disengaging.