It's the same one that was resealed in March, rear left. Fortunately, I had my marvelous cigarette lighter powered tyre pump, so I was able to pump it up and drive home. It seems to be staying up for the time being, but I am keeping a wary eye on it.
WHY DOES THE GOD OF TYRES HATE ME, WHY???
Also, I have just realised that I drove to Oxfordshire and back in a car without a current MOT. Oopsie. Booked in now.