I think the chicks have hatched today.
I noticed that when one of the adults would fly to the nest to take over nesting duty that the bird bought a small parcel of food in its beak. When arriving at the nest for change over the sitting bird would take off and the bird arriving either delivered a small parcel of food into the nest or pecked around in the nest as if to clean or arrange the nesting material. This bird would sit on the nest for a short while until its partner arrived to take over, repeating the process.
At this stage the chicks are too small to see or hear in the nest.