A territorial dispute...

... was in full swing when we arrived on the scene.

This nuthatch clearly expected squatters rights to the hole in the tree, but the great tit (in my poor quality extra) popped its head out of the hole and battle commenced. 

Unfortunately, I couldn't capture the in-flight battle as it raged behind some thick branches. The nuthatch then took up its sentry post at the entrance to the hole, occasionally giving chase to the potential squatter.
I didn't manage a better shot of the great tit, because it didn't get anywhere near again.

I might re-visit to see how the situation progresses.