I've checked the hanger and that seems solid. There is some movement/wobble in the derailleur at the bolt that connects it to the hanger, like it's not quite tight enough even though it's tightened as far as it will go. When followed down to the cage it feels like 1-2mm in sideways cage movement, maybe more - it feels significant anyway. The cage itself is definitely still solid on the derailleur, and it lines up straight under all of the sprockets.
I took the derailleur off, cleaned the threads and everything around them including the c-clip, refitted but still some wobble. I think it's affecting my shifting as the cage might not necessarily return to the exact same place at the extremes of the cassette meaning I can't find a cable tension that works for all gears.
Is there anything else I can check before trying a new derailleur? (I have a medium cage ready to go but the current short cage has been working fine so reluctant just to retire it)
I did notice that around the c-clip it wasn't an entirely flat surface, there were a few metallic ridges the were higher around the edge than other parts of the surface. Are they there for a reason and do they have to be located in a certain place? It's a Tiagra derailleur, looks pretty much like this: https://i.stack.imgur.com/LsyF4.jpg