What I had to do to edit the SR was to load the SR into iD by editing it from the website, then moving to https://www.openstreetmap.org/#map=20/53.3102525/-1.8310127 where both stages 1 and 2 are.  From stage 2 (which was a member of the SR) I could select the SR.  I could then go to stage 1 and add to the SR by number.