Boxkite overflight
As a sprog I lived at Kenley in Surrey (in the valley) and was woken at some ludicrous early hour by a loud engine noise that beamed from the sky but seemed stationary. Seconds later the 'Boxkite' hove into view at a very low height and passed overhead at a very slow speed.
I suspect it was 'in transit' back home after an appearance at a Biggin Hill airshow or RAF 'At Home', making good use of the early morning calm, no thermals and low temp, which was needed to defy gravity.