I think we may also have two robins. Certainly there are two small birds of the right size. I've seen one chasing the other. Just now I picked up two robins in a sweep of my field glasses- one perched in the holly bush and the other hopping about on the ground. Only I can't be sure it wasn't the same robin. They move awfully fast.
Bingo! It's settled, I've just caught two robins side by side on the ground.