Thanks for the reply.
The roller block is good and there are no cracks or any visible damage to the coupling. The magnet however in the coupling does look very slighlty out of true.
It all looks good and true until I tighten the coupling retaing screw, then as I tighten it I can clearly see it being pulled to an off square angle. I am almost sure it's the little brass retainer for the brass collet, or the brass collet itself that is worn. I can't really do much to shim this part.
I reckon my best bet is just to buy a new coupling and hope for the best.