So I got a "bargain" pair of SLX UST wheels from Decathlon (I know!).
Anyway the symptoms started as shifting all over the place / ghost shifting when on the bike: but on the stand it's perfect. A bit more digging and I thought the cassette was loose - but it definitely isnt. Basically the cassesse moves in/out from the hub by 1-2mm, took the cassette off and its the freehub moving.
The cones are tight - any tighter and the hub is notchy.
I've never done anything to Shimano hubs apart from servicing them and adjusting cones. I've checked Sheldon but nothing obvious.
Apologies if its something obvious! Oh - and I'm loathe to bring them back to Decathlon- I'd prefer to learn as I fix.