Mine looks the same as that picture, moves very smoothly but is stiff even without the cable attatched and off the bike
Ok now we know what the problem is. What you need is spray on grease, something like spray on lithium grease and a can or WD40 Available in Halfords or local bike shop or auto factors. At least Halfords are open on Sunday
Before you strip any thing, remove the rubber bung, loosen the lock nut and remove it then turn the screw in full..
Remove the sprocket cover and remove the clutch cable.
Place the cover inside down on a flat surface with a few rags under it.
Fill the hole where you removed the locknut and screw with WD40 leave it for 15 minutes, There is a seal on lever inside to keep the grease in, but the WD will probably leak out slowly. Keep the hole filled to allow the WD$) to soak through the corrosion.
Try operating the lever and it should get progressively easier. Repeat the process until it is free moving. When movement is free remove the internal screw and let the WD40 drain as much as possible.
Now follow the instructions posted her
http://foc-u.co.uk/index.php?action=downloads;sa=view;down=60