"have you watched steptoe recently? i thought i was going to open a vein... "
The horse-drawn carriage was there to deliver the punch lines, very slowly and very deliberately, signalling their arrival with a clippity clop.
It's likely that, in fact, Galton and Simpson ultimately don't stand examination. Most of Hancock's work is about him (ie, before he got lazy and started to read everything off boards), and that desperate series in which Paul Merton used all of his not inconsiderable talent to demonstrate that the scripts aren't, in fact, funny rather proved that.
"without a good script, Joanna Lumley would have had no basis for her brilliant performance now would she?"
Was it a good script? I'm not competent to analyse it, but were Saunders to be able to write well, there would have been a joke somewhere in the past twenty years of French and Saunders, wouldn't there? I mean, just the one would be proof she could do it occasionally. Whereas Ab Fab is actually funny, mostly when Lumley is on the screen, and desperate beyond words when she isn't. Either Joanna had her own writer, who was prepared to remain uncredited in exchange for kisses, or she was able to get the best out of ropey material. That superb BBC2 thing she did a few years ago (Up in town?) proves she has amazing timing.
"it just got all those characters into a recognisable space."
You could be right. To me, I couldn't really get past the fact that I kept on expecting George Formby to come on and ask us to buy war bonds.