Hope pro 2 with an XTR cassette fitted. New wheel, old cassette. It has developed a load of sideways play between the higher and lower cogs. In fact its knocking and clanging like hell over rough ground. Has anyone fitted shims between the mating surfaces?

afair an xtr cassette needs a spacer behind it like a Dura Ace cassette does

Spacer? Does this go on the inside or are you meaning the shim on the locking nut?
When I say play I mean in the direction of the chain travel. The main body has a spider and the smaller gears are loose. Its the spider part that's rocking front to rear.

When you tighten up the lockring there should be no movement on the cassette / sprockets in relation the the freehub.
Halfbee may have the answer….

Other possible is that the lockring isn't done up tight enough – but surely that's too obvious?

The lockring is taking out the top to bottom play but it is not clamping the gears together. I will google halfbee's spacer idea or try another shim.

Cheers for pic, halfbee.
Lockring is torqued up to correct setting.

I guess your lock ring is bottoming out without locking the cassette together proerly

Yes that seems to be the problem with my new wheel-set. All was ok with the old wheel.