So they normally need 6 people to keep 4 positions open?
Positions being: Ground; Tower; Approach/Radar; and an assistant?
I'm guessing that on reduced staff they combine ground and tower (like they do at night normally anyway) so the tower controller no longer has capacity to fill in the electronic board when you call up "blind" (watching them do this is painful - based GA traffic have their own "shortcuts" on the board but it takes time and is distracting to use a pen to select in all the details of new traffic) while if you phone an assistant can have the virtual "strip" ready to go.
I wonder how long it takes the assistant to fill in the details - I wonder if I could phone up while airborne. I also wonder if they could hook the board into Mode-S, so the board could be ready with a shortcut for any detected aircraft?
Glasgow has (in the past) been great for me too - I've had no issue with previous transits.