Clutch Drag 175 Silverstone Super

I am suffering from what appears to be clutch drag on my Silverstone Super 175. It gets worse as the bike gets hotter. When it first happened we examined the clutch and the centre nut had come loose. Fixed that and rebuilt it but after a ride yesterday I still have the same issue. It will coast along at 10 mph with the clutch lever fully pulled in. Any ideas appreciated.