Alan,

It's most definitely a Damselfly. Depending on where you are geographically it may be a Demoiselle. Damselflies spend most of their time as entirely aquatic nymphs but emerge for a brief adult phase when they devour huge numbers of smaller flying insects.

They also spend long periods sunbathing (particularly in the early mornings) to build up their body temperatures for flight and will be more or less dormant in the shelter of trees and shrubs on cool, overcast or rainy days.