I'm sitting here, attempting to concentrate, and I keep getting distracted by the sound of aero engines.
Ah, I apologise, they're not warbirds after all. I just got the binoculars out and checked, and that group of four aircraft doing formation loops have fixed undercarriages and look to be low wing monoplane trainers. They're behaving like warbirds, though. Right now all four of them are diving towards the same bit of sky from four different directions.
I assume they're practising for Duxford this weekend, but they're not on the guest list.
(Why binoculars at my desk? Well, I have a window seat, and a nice large chunk of sky beyond it, and it's not infrequent that there's somebody out in it doing aerobatics. And we have had a hawk sitting on the car park fence before now.)