Name that bird?

I'm fed up taking pictures of the birds through the kitchen window, so today I lurked around outside long enough for a few to become comfortable with me being near(ish) and start to gather around the nuts and other goodies.

We mainly had the bluetits today, but there were several of these guys hopping around. They're close enough to spherical that they could almost have been drawn for Angry Birds, and (as with most birds) I don't have a clue what they are. Can anyone enlighten me, please?