I have always assumed that I was seeing Common Blue Damselflies around the garden pond but on close examination of the markings on one of the two seen today it turned out to be a male Azure Damselfly:
Most noticeable for identification of the Azure is the thin U shaped black marking on a segment behind its wings:
One excellent source for identification is the Guide to Dragonflies and Damselflies of Britain published by FSC Publications. A well illustrated laminated 12 page fold out guide which can be purchased from around £2.
I will have to check more carefully during the rest of the year as, hopefully, a few more appear.