Must be this one overhead me here in the garden right now.
Meridian Air Cargo An-12 UR-CIC, 21,000 ft, Liverpool to Paris, headed due east across Central London (hmmm, that's not the way from Liverpool to Paris - but whatever). For ONCE perfectly silhouetted at altitude against high cirrus. Heard it coming, no computer needed, looked up and there it was. Classic drone from four, ever so slightly unsynchronised props.
Meanwhile, on this pleasant warm summer evening, the rest of the family just carried on eating ... Come on Little Miss WHBM, aged 9, it's an Antonov, you don't see one every day, up there (points), look up ... um ... no.