As promised, I'm reporting back now I've had chance to have a proper go.
Where I thought the cable outer was split, it turned out once I could get at it that it was just where the extra bit of plastic tubing ended that the cable routes through, the cable itself was fine. I took the clutch lever apart and tried moving the inner cable while holding the sheath. It moved so it didn't look like the cable was frayed internally. Following the hints and tips on here, I slowly dripped 3-in-1 oil down the cable and kept moving the inner to work the oil in. I did that for about 10 minutes then left it overnight to run down the cable. I greased the clutch handle pivot and put it back together, it is now a joy to use.
I've got a spare cable now for the next time though
Thank you all for your help, it was greatly appreciated.
This forum is brilliant, I now have the workshop manuals to enable me to tackle the next job, the one where the ABS light never goes out. I'll see you all when I post for help on that one