I decided to get on with a job that needed doing in order to take my mind off of Swifts. After about half an hour I needed a break so I went for another sit on the balcony. A few minutes later a Swift entered my box, so I abandoned the job I had been doing and resumed "Swift watch." I sat for quite a long time with my camera focused on the entrance to the box and eventually I detected movement in the shadowy interior.
But I couldn't actually see what was going on, it wasn't until I came indoors to download my photos that I realised I had managed to get a couple of shots of two of the little beauties. Not a very good quality picture again I'm afraid, but I'm pleased with what it shows.