I feel a bit stupid for not taking a picture of the whole plant as it is a massive thing towering to around two metres. There were several of them spaced out in a glade amongst pines behind the beach at Burry Port on the north side of the Loughor estuary. They looked like a weird installation.
The book says they are strong smelling, so I crushed a bit to check. Later it warns you that the sap is highly irritant, causing a nasty rash on exposure to sunlight. I can't help thinking the warning should have come first.
It is Wild Parsnip, Pastinaca sativa. It is generally classified as the same species as the vegetable but a number of different subspecies are recognised.
The plant contains useful chemicals, including coumarins and psoralens. The latter absorb UV light and are used in suncreams.