After a bit of internet searching and help from Ted Benton, they turned out to be Willow Emerald damselflies, perhaps not that unusual on this river, but there were at least 4 of them and my photographs revealed the pair to be engaged in ovipositing. Like the Willow, the Elm tree has soft bark, that this damselfly needs, in which to lays their eggs, provided that the branches are overhanging the water. The larvae stay within the egg over winter and then hatch out in spring, falling to the water. The life-cycle is very quick, as they reach maturity in late summer/autumn.
My internet searching also revealed that this species has recently been seen at Rainham Marshes for the first time, and also a single specimen in Hertfordshire.