I head something turn and drone over the south Croydon area around that time but it was pretty cloudy and I couldn't see it - I thought it was heading more north easterly. Sounded rather like a Transall, but I guess Orions/Electras may sound similar, though I would have expected them to sound more like a Hercules.
By the way, Atlantics are twins!
Talking of Transalls, hadn't seen one of those for a long time but last summer while working in the garden I heard a familar drone, nipped inside and grabbed the bins - yep!