I'm off out for a few day over the Jubilee weekend, so I thought I'd better sort it out.

Turns out that it was a problem between keyboard and chair, as per. I thought it was a clutch issue, i.e. it just needed tightened, but it turns out I was wrong. I dismantled the derailleur I knackered last Spring, so I could have a look at the clutch and see why that one was working, but this one wasn't. When I took the bent cage off the old derailleur, I noticed that there was a washer over the cage bolt, I couldn't remember fitting one to the Garbaruk cage. So it turns out that for some reason I hadn't transferred the washer from the old cage, to the new cage. The moment I put the washer in place and screwed it all back together, it started working as expected.
With the clutch now working and the cage moving freely, I hope that's the end of the issues.
