Oat / Cultivated Oat
Flower-heads with large spikelets hanging from slender stalks, each with 2-3 flowers. Short-toothed outer flower bract (lemma) without tuft of hairs at base. Flat leaves with split sheaths. Stems to 120cm.
Common and widespread casual as relic of, or escape from cultivation but rarely persistent.
Similar: Wild Oat, Avena fatua