Cerastium glomeratum Sticky Mouse-ear CCC DD N
Not quite as common as C. fontanum in my experience but this plant is still easy to find. Although the name suggests you should be looking for the stickiness of the plant, the immediate and obvious difference between this an C. fontanum is that the flowers are in compact bunches at the end of the stem. To be sure, touch the flower head and you will then feel the stickiness.
Apart from the tops of the highest mountains this plant is found everywhere in the British Isles and Ireland.