We have confirmation that there are definitely chicks in the nest – a pale yellow head has peeked out of the entrance hole! The juvenile markings are suffiently different to the adults and showed that this was clearly a youngster, not a parent! (The youngsters have a much yellower face than the adult’s white and black, and the beak is yellow not black).
We’ve started seeing an occasional changeover of parents, where both parents were entering and exiting the nest box at the same time. And on arrival, at least one of the parents is often tending to sit away on nearby branches for a while first, calling to the young before going into the nest-box entrance hole.
We’ve also seen a new activity – parents bringing out poo-sacs from the young in the nest. They deposit these some distance from the nest to keep it clean and to help discourage predators from smelling the vulnerable young.
Update: 28th May 2020. We realise now that the increasing amount of time the blue-tit parents were spending sitting in branches near the nest-box and at the edge of the box with food was an attempt to encourage the chicks out and to fledge. Their calls were getting increasingly strident as if to say “Dinner’s out here, stop being lazy and come and get it!”
So it seems our chicks were about to fledge and probably the nest box was occupied long before we were aware of it!