Noise from clutch?
 

2001 Fs1, 35000 miles. Came back from a 3hr ride the other day, took out my ear plugs with the bike idling and there's a random, lumpy, knocking noise coming from the back of the engine. Not heard it before, doesn't sound like clutch rattle - it's a heavier noise.... bearing? It disappears when clutch lever pulled.
I've just taken off the clutch cover and taken out the plates. All look good, measured thickness, virtually no wear. Measured springs, all good. Almost no wear in grooves in clutch centre or clutch housing. Outer bearing nice and smooth.
There's about 3 or 4mm stiff movement in the clutch housing when I try to turn it. Clutch centre spins freely with no side play or noise but what is odd is when I put the bike in gear there's almost ¼ turn play when I turn it. Seems a huge amount.
I've tried to take off the clutch nut with an extended bar and holding the rear brake hard, then tried an impact driver - both unsuccessful. I would like to examine the needle bearing and the splines on the shaft. Any advice on how to get the nut off, and also what the noise might be?