I am editing an existing bus service whose end point has changed. As soon as I move off its existing route, the service doesn’t appear in the drop-down relation list - so I can’t add the new route to the relevant links. Doubtless there’s a way to fix!
Assuming iD, it shows you only the relations in your current “view”.
You could select the element, then go back to the relation area so it appears on the list.
Or you could copy the relation identifier, then instead of searching it in the list, paste the value (in the “r1234567” format), it should work (just tried).
I thought this had resolved it - but alas not. I’m using iD in Edge, there doesn’t appear to be a place where I can find the relation ref to paste into the ‘new relation’ field for the link in question. In iD I can certainly call up all the points which have the relation applied to it (it’s easy - it’s a bus route) but no-where I can see if there a place where it says ‘and this is the reference code for this relation’ which would be ideal. And on returning to the mapping view, the list of relations no longer has the route in question in the drop down. A doom loop! Could you lead me through where to find this please? Really appreciated.
When you select a relation (or any other object) in iD, there is an “View in openstreetmap.org” link at the bottom of the left sidebar:
So you can open it and copy the last part of the URL, or, perhaps faster, hover the mouse to display the tooltip and remember the 7-8 digit identifier.
I will agree that it’s a quite roundabout method, but iD developers are always wary of displaying raw IDs directly in the UI, lest they “confuse the newbies”. 
Thanks for the advice, I clicked the link to Open StreetMap for the relation, copied the relation code (2284638), went back to iD and pasted the code into the ‘add to a relation > chose a parent relation’ box. It appeared there, but as soon as I move away from the box, it disappeared. IT JUST WON’T STICK arggghhhh