Polypogon monspeliensis Annual Beard Grass RR N
This is one of those rare occasions where I have to disagree with a Stace 4 estimate. In New Flora of the British Isles Edition 4 this grass is give RR status which means it isn't found in any more than 100 different 10 x 10km squares in the Britihs isles since 1987.
Along with a grass in the same genus, Polypogon viridis (Water Bent), this native grass has spread from the south of England and is not at all unusual in the north. Both on waste ground and in the gutters and pavement cracks this distinctive grass thrives.