Our bird (now known as Strange Orange Bird) has been around a lot, sometimes sitting on the windowledge, looking into the kitchen and singing loudly. We have had more pictures, but nothing that added to anything - until today.
I can now see that the SOB definitely is ringed, so must be an escapee. Given the limitations of our camera I don't think we can do much better than this, which is through the kitchen window.
All the evidence points to some kind of canary, although I don't know exactly which variety. Whatever it is, we are really pleased to have it as a regular visitor to our garden at the moment.
Once again, thanks to everyone who helped with the identification of our SOB