This episode is very much about me, a series and a half in, experimenting with splitting the foursome up in a different way from the usual flighdeck / cabin crew division. Interestingly (to me, anyway) the effect of that is that it becomes an episode all about Douglas. The Martin / Arthur story is all about the absence of Douglasn- what happens to Martin when, on the one hand, he doesn't have to worry about being constantly put down; but on the other hand, he can't rely on Douglas to find the clever solution and sort out any problem. And the answer the episode seems to provide is that he grows in confidence, and finds those solutions for himself - and after all, as early as Boston we saw him 'pull a Douglas' in the authoritative and creative way he persuades the paramedics to take Leeman away.
Meanwhile, back at the airfield Carolyn and Douglas are experiencing a more straight-forward 'Freaky Friday' style role reversal, with Douglas discovering what it's like to really care about the success or failure of the company, and Carolyn enjoying getting to play the Douglas role, and 'chip in from the sidelines'. Overall, looking back at it now, it's a surprisingly anti-Douglas episode, certainly more than I realised at the time. Even Arthur, who famously thinks everyone is brilliant, guiltily confesses he's enjoying the 'holiday' from sarcasm. And then look at the final scene, and how Douglas is torn (and Roger, of course, plays this perfectly): he clearly really wants to take off on time, and thus beat Carolyn; but he also really wants Martin to fail, so as not to disturb the flight deck hierarchy. I would say that over the twenty-six episodes, Martin is the character who changes most obviously; but there's also no way Douglas would have behaved the same way had the events of Jo'burg taken place in series four.
All of which is not to say I don't like Johannesburg- I actually like it very much. I think it's got one of the most cunningly laid endings to any episode except maybe St Petersburg - I'm sure many first time listeners wondered why I kept stressing that the newly washed BMW was parked behind the plane, but I like to think most of them will have forgotten the wine bottle on the engine exhaust, even when they hear the smash.
I also like it that both the Spanish airport characters are, I think, funny- but neither is in any way obstructive or unhelpful, a la Douz or Newcastle. They both just do their job.
(If you think this has all got a bit back-slappy and self congratulatory, by the way, do not fear - the antidote is on its way. My least favourite episode is just around the corner...)