Daffy – I had the exact same problem with my King rear hub.
Back pedalling gave some resistance so the chain would slap around and when I did the QR up then the cassette was really stiff, was fine with QR undone.
Finally found the problem when I examined the hub closely. The drive side axle end (the pressed on bit with the serrations on) had fractured and parts of the metal had got into the freehub mechanism. When the QR was done up it caused the piece of metal to jam the freehub. It was new axle time and although the hub was > 5 yrs old King warrantied it no probs and sent a new axle out.
Worth checking the drive side of your axle and making sure the same thing hasn’t happened to you ?