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So I fitted the second stay. There isn't space for the mount (and nut) on the inside, at least on my driveside (YMMV as I have an alfine). Stacked two up on the outside and it works well enough, bit of pushing against the rack but nothing major. Might put a piece of rubber/insulating tape in there to stop it rattling/rubbing/fretting too badly.
Will report back as to whether it stop the rattling in the rear mudguard, hopefully it has as it spoils an otherwise quiet bike!
The extra stays arrived today. I was thinking of fitting one on the outside and one on the inside (with a nut). I haven't got as far as thinking about clearance...
Will investigate and report back!