It must have been very expensive to operate - a second aeroplane and crew standing by just in case the first one filled up. I remember it well at Manchester, operated by Tridents and 1-11s.
I remember 'back in the day' (seems odd to say that about the 80s) there was a story that on just one occasion only a Concorde was used as a reserve on one of the shuttles, either the Manchester or Glasgow one. If true, it must have cost a few bob.