There is a possibillity, remote as it is, that the airport owners have given permission for the MOD to use the airfield out of hours.
At civillian airports I've worked at in the past, this has taken place (and still does), the a/c are usually C130s and operated on a lights out/NVG basis.
I once waited up for them and aside from an initial disturbance there was hardly any noise & you'd certainly never know they'd been unless you'd seen them.
I know that Biggin is noise sensitive etc, but this sort of activity is not usually subject to planning by laws, so they could get away with it.
Luigi
HD may be old, but I'm sure that his nuts are probably far more experienced than yours and that they haven't shrivelled up just yet
