They’re back.

I blipped recently that I was confused as to why the sparrows were trying to get into the box when we were pretty sure the blue tits had already built their nest. 

Google to the rescue again.

 It is a well known fact that sparrows raid blue tit nesting boxes 
particularly if the entrance hole has been widened by years of use. They destroy the eggs and often kill the young chicks. 

The image I missed by seconds was the blue tit returning and trying to chase the sparrows away but because of the size difference it lost out and flew off. 

Next year I will invest in a new box, probably one with a metal rim around the hole to stop it widening again.