They've got their passengers captive for 10-12 hours or so, not difficult to get them to fill in a questionnaire.
Very true, Stansted seems an odd place to connect though, there's no Flight Connections option and you have to clear security and recheck all baggage as well make sure your connection is on time. Gatwick will have the same problems as you can only use Flight Connections with checked baggage so unless they're interlining with the locos with through checking of luggage, it's no more attractive than being in Essex. Perhaps the fall in connecting traffic is down to their being easier ways to connect to long haul than AirAsiaX and the market coming to realise this with the knock on effect on repeat traffic?