Bird Nest Box !

I am throwing down a challenge, inside the centre post of this steel gate set, accessable only through the latch slots, Blue Tits nest! I had thought that it had been left vaccant this year but today I heard the chicks calling to be fed.
The gate is on a busy route through the yard leading to the geldings turnout field and the muck hill, the other thing I struggle with each time I hear them in there is how do they get out on fledging? when you open the gate and try to look in they are low enough to be out of sight, and how do the adult get back up, they can not use thier wings as the pipe is not big enough! Anyway my challenge, blip a stranger nest box if you can!!!!!