I thought Tawny owl, they sound like that over our house then another usually answers with a twit-twit.
Best wishes
Val.
The tone is right for a short eared owl, but it sounds like it hasn't quite learned the full call! They do pass through London along with many other weird and wonderful migrants.
Just heard this exact same noise while out the back of our house in N22 at around midnight. Thought it was an owl, until I heard the customary rustling noise in the little passage that separates the back gardens all along our road(s) - a fox making its usual rounds; and then that noise again, from the direction in which the fox had just travelled. Their range of calls must be extensive, to encompass such a muted call among their usual shrill "vocabulary".
I'm no expert at all - so take what I say with a pinch of salt. But I literally just listened (an hour ago) to the chapter on tawny owls in 'Undiscovered Owls' by The Sound Approach (brilliant, brilliant book by the way). And it doesn't sound anything like the tawnies they have recorded. I don't think it's a shortie either. But like I say, not an expert! I'm following this feeling intrigued to find out what it is!