I’ve taken it all apart again and double checked everything is present and correct. It is. Plus all bearings are ok.
I have noticed my hanger is a little out of alignment, so my new thought is that perhaps the RD is a bit off and this is causing the issue. I’ve got a new derailleur coming and have straightened the hanger by eye, so will see if that’s the culprit.
If not, I will be setting fire to the whole thing.