Some recordings have “live recording of” relationship links to the original studio recording.
E.g., Recording “Barracuda” by Heart - MusicBrainz
That isn’t in the drop-down I get for adding relationships to a recording, so how can I add one? Or can I not, now?
E.g., for Recording “How Can I Refuse” by Heart - MusicBrainz
Hi!
Those aren’t links to recordings, but to works. You should add a recording-work relationship - and if the work doesn’t exist, you’ll need to create it first 
That’s a relationship to the work (song)
So if the work exists you can add to that, if not you can add the work and relationships to its composers and all the other recordings of it e.g.
http://musicbrainz.org/work/66aecb42-f538-408a-bfb1-ca47c7383da2
d’ah got there before me 
OK, thanks everyone. I was wondering if that was the route.
Maybe there should be a “make work of this recording” shortcut? There looks like there are an awful lot of separate recordings that ought otherwise to to be related to a work.
As it happens, there is: The relationship editor for releases has a bulk-add feature.
Although it doesn’t add relationships with all the other recordings that should be related to the work to it.
P.S. Not suggesting it should, just clarifying what it does
I see 3 batch add links along the top of the relationship editor, but they’re all greyed out.
Have you selected any of the tracks?