The Return of the Great Tits?

I don't know if you remember that four years ago we had great tits nesting in our mail box?
I've heard them calling for about a week and today spotted one coming and going from the box. This was taken through the kitchen window before I spotted them to-ing and fro-ing.

Are they just checking out suitable nesting sites or are they moving in? Who knows, but I'll definitely keep you posted! (groan)