Oxford Ragwort

Senecio squalidus

An introduced flower from Sicily, where it grew on the volcanic slopes. It was planted as an exotic species at the Oxford Botanic Gardens in 1719, then became naturalised in the area (getting a foothold in the stonework of the Bodleian Library), hence the name.
In Victorian times, it spread throughout the UK, spreading up the railway lines, where it liked the conditions on the rocky ballast around the rails, and its seeds were helped to spread by the slipstream of the trains.