I went up to the park after work to see if there were any cygnets. At first I couldn't see the swans at all and I thought that they had left the nest. Eventually I spotted them hidden in the reeds with one cygnet.
I talked, at a distance, to a few other people watching. One said they'd heard there were 3-4 eggs, another said there had been at least two cygnets hatched, one said they'd heard the heron had taken a cygnet.
The cygnet was too far away to get a photo so here's a fuzzy shot of the heron that may have reduced the family's numbers. The coot is still sitting on its nest.