Just had an email from the brilliant Jim Hardie (Royal Entomological Society) telling me they are eggs of horse flies from the family Tabanidae and when the larvae hatch they will fall into the water where they will develop. This makes complete sense now as I have photographed several horse flies by that pond...
This one was taken at that site on 03/08/19
Dark Giant Horsefly [Tabanus sudeticus]
2013 photos & vids here
eff37 on Flickr