I was able to photograph both males and females. This is a female, somewhat cryptically coloured. The males were blue, with dark, smoky-looking wings:
Link - altogether more conspicuous.
I know very little about damselflies, and initially identified the colony at Lemsford as Agrion virgo. Hugh Venables - who knows a lot more about damselflies than I do - assures me that this is A. splendens rather than A. virgo. My initial identification took into consideration habitat at Lemsford - clear water flowing over gravelly and sandy stream-beds - which might suggest A. virgo.
The site protected by Lemsford Springs nature reserve includes old watercress beds.