[Catrin]

Having heard it singing from a neighbour's TV antenna this afternoon I'm quite prepared to believe that it is some kind of canary - a very complex song with lots of variation and trills - however, the back does seem to suggest that it is a cross with something else.

I didn't know that canaries could cross with finches, however, a quick google has told me that they are commonly crossed in Europe with goldfinches, siskins and greenfinches - I now understand why you are referring to it as a 'mule'.

Thanks for everyone's suggestions. Sorry, but we haven't been able to take any more photos of it this afternoon.