This is a common blue damselfly but is not THE common blue damselfly! There is a tiny difference between the common blue and the azure damselfly (Coenagrion puella) and closer inspection of the first segment of the abdomen will reveal that this is the azure damselfly rather than the common blue. Why? It has a black tumbler shape on this segment whereas the common blue damselfly has a wine glass!
There are other differences to be fair but telling them apart in the field is not easy. The common blue has more black patches on the blue abdomen than the azure and the azure is more numerous in May and June whereas the common blue is more numerous later in the summer. The azure tends to be found around small, sheltered sites such as ditches and garden ponds whereas the common blue prefers larger, more open areas of water. You would rarely see a common blue around your garden pond for example, much more likely to be azure.
All in all, though, tricky chaps!