Took some photos, but they don't show it that well as the mechanic painted over the sprocket, so the join is not obvious.
I had to move the chain guard out of the way (rust patch found on guard mounting bracket!) - the bolt is one of the 4 holding the sprocket in place, while the other smaller hole is one of the 6 bolt holes from the new sprocket.
The paint was flaking badly on my swing arm so I did a quick touch up before my trip - I want to try and get an aluminium replacement as Chinese paintwork on metal (maybe just on Ronco's??) is not brilliant!
Edit: OK, I can't count - looks like the new (outer) sprocket has 8 bolt holes, not 6!