Can anyone suggest what might be wrong? I have just changed the clutch and now I am struggling to select gears even when the engine isn't running.

I don't think it's linkage related because it seems to be difficult to select gears just pulling directly on the arms that move the forks too. I don't think I've done anything that should have an effect on the inside of the gearbox, I've removed and refitted the drive shafts and changed the fluid.

Also can anyone confirm my thinking that the clutch and concentric slave cylinder shouldn't be a factor, since the problem exists when the engine isn't running?

Edit: Strangely, after leaving it overnight, gears are selecting perfectly again and I have absolutely no idea why. I didn't change anything 😕

I suppose that should be good news but without knowing what the problem was, I'm not very satisfied. Thank you to everyone for your help.