I had to get very close, but the flower does have a strong scent , supporting Ashley's identification.
I did order some primula seeds in April from Plant World Seeds, including P florindae variety "Ray's Ruby" - but that has red flowers, so it couldn't be that, even if they had germinated, which they haven't. (There's time yet...)
In any case, the rest of the cowslips in the planter were planted out last autumn.
Maybe a passing bird?? It would take precision bombing. More probably a mislabelled seed I expect. Most welcome anyway. I'll definitely try collecting any seed.
A