Carrion Crow
Corvus corone
The evenly spaced, three, four or five-note call of the carrion crow is usually delivered from a high perch.
From a distance, and especially in flight, the carrion crow looks similar to our other big, common crow, the rook. However, the latter tend to be much more gregarious, and of course nest communally (often alongside Jackdaws).
If you see a solo crow, or a pair of birds, the chances are very good that they are carrion crows.