The radars are still being recorded overnight.
Get some accurate times from those doing the complaining then ask your manager to ask TC if they can check the replay for anything strange going on.
Unless it's somewhat stealthy it should show up.
In the last few weeks there have been various calibration a/c for various services (as in ILS or VOR, not military) flying past Biggin during the night period but they're not large, not noisy and they don't go down to 50ft. Well not over Biggin anyway.
Whilst those of us that do nightshifts at TC on the approach sectors don't necessarily stare fixatedly at the radar if we don't have to i.e. none of our own traffic about, we are looking at it often enough to notice unusual goings on and I haven't myself seen nor heard anyone else mention such shenanigans.