Germander Speedwell (Veronica chamaedrys)
When I was young we used to know this plant as 'Birds-eye'. It is now better known as 'Speedwell' I think, but my field guide lists fifteen species of Speedwell so how do you know which one this is?
Looking just at the flower alone is not going to get you to the answer! In general, Germander Speedwell is a darker blue than many of the family but this is not always the case.
About a half of the Speedwell's have a flower the shape of these, with three small lobed petals on top and a single longer one below.
There are three other factors you can take in to account however to help you decide. Firstly, and most importantly, the leaves. Germander grows in small 'bushes' and has many leaves with a slightly serrated edge. Secondly, it grows in many environments; woods, hedgerows and on grassland, whereas other species are more habitat selective and many are weeds of cultivation.
Finally, of course, the Germander Speedwell is by far the most common of the family as well as flowering from March to July so, if you see a Speedwell then decide first if it is not Germander before thinking about what it might be!