I decided to change the cable on my relatively new [6mths] left SLX shifter as the cable was beginning to fray alarmingly. However I have been unable to get a new cable to work. I have threaded through to the mech OK but when you try to shift up the shifter makes a graunchy noise as if slipping. I have taken it to bits as far as I dare but can see nothing obvious wrong. Any bright ideas gratefully received.

When you threaded the cable through, did you make sure the shifter was in the lowest tension position (ie. the same as if you were in the smallest sprocket)? Can you feel the change in tension if you hold the cable and adjust the shifter without attaching to the mech?

Yes I did this and you could feel the change in tension until after the graunch when the pushing the levers did nothing!