It was, in fact, a robin. They are one of the few birds that sing in the winter when other birds have stopped and they do this to hold territory. Their territory is very important to them so that they can access to enough food to survive - it can be a matter of life and death.
They may be the pin-up bird of Christmas, but they can be very aggressive and attack other robins if they encroach on their territory. Not particularly festive spirited of them!
Adult male and female robins have the famous red patch on their breast which they puff out in territorial display. Robins have even been known to viciously attack stuffed red robins or even material that is red and a similar size to a bird.
So why was it singing at night? Well, the answer here may be that I have a street light outside my house - a source of light prompting the bird to sing. Robins rely heavily on their territory for survival and will work all hours to keep it!
In the festive spirit I'm loving this Breathing Places song. Nothing quite like seeing human mouths on animals!