Well firstly, lucky you, i'd love to see that happen...where are the photos!! I'd be there for hours with my camera
My pond is fairly new so can't help you there - I have larvae but won't hatch for a year or two.
I can't advise on Common Darter but on Wednesday this week there were a HUGE number of hawkers in the air at Brede High Woods in Sussex. I would estimate around 200 or so with exaggerating, all around a relatively small heathland area.
TobyH