Large number of these insects swarming around a fishpond, but no obvious nest
No interactions present.
They look like workers collecting drinking water for there brood and queen.
The water looks like it has evaporated to leave weed exposed and they are all lined up with there heads down collecting the water from the still wet weed.
They are only going to need to do this if there is very dry weather at the time.
Lat/Lng: 52.0, -0.9
OS grid ref: SP8138