Both Cowslips (Primula veris) and Oxlips (Primula elatior) have tall upright stalks with clusters of bell-shaped flowers dangling from a central stem, and of the two, the flowers on oxlips are a deeper yellow.
Primroses (Primula vulgaris), have single, open flowers – which I think makes this a primrose. I have seen cowslips and oxlips, so I think I am right – but then this from Gardening Direct.
What are polyanthus plants?
Polyanthus flowers appear singularly, clustered in a rosette at tI base; some varieties also grow on long, stalked umbels.
Polyanthus is thought to have originated from a hybrid between the cowslip (Primula veris) and the common primrose (Primula vulgaris) which are familiar from English hedgerows and verges. The resulting hybrid was called a ‘false oxlip’, and it began to be appreciated for its own showier flowers. The word ‘Polyanthus’ began to be used in the 1670s, with a labelled illustration first appearing in a plant catalogue in 1687.
Photograph Works 