Eriophorum - Cottongrasses
Eriophorum angustifolium - Common Cottongrass
It has more than one flower-head and tends to grow singly. Roll the stem just below the flower head, it is three-sided but lower down the stem it is round.
Eriophorum vaginatum - Hare's-tail Cottongrass
This has only one flower-head, and grows in clumps. The stem is round from below the flower all the way down.