Righto keg and Captn Claret et al. Thanks, that clears up my query.
I guess my original worry was hearing that an airline possibly had pax sitting on the ground sweltering without air. An airline would do that at it's own peril I suppose.
There's been plenty of publicity about leaving pets and babies in cars on hot days or any day for that matter and the potential for overheating and even death.
I don't imagine that APU's break down that often anyway.