It is one of the ragworts.
These used to be classified in the genus Senecio, but at least some of them have been moved into Jacobaea (various sources, including the highly respected The Plant List). Confusingly, the BSBI still has them all in Senecio. Go to http://www.bsbimaps.org.uk/atlas/main.php, put 'Senecio' into the second box, click 'Find taxon'.
To be honest, I'm not sure which species you have, but there's a good chance it is common ragwort, Senecio jacobaea (= Jacobaea vulgaris)
Whatever its name, I am sure the cinnabar moth caterpillars will love it.
If you don't know about them, so a little research then keep an eye out.
And if you find them, watch out for the adult moth later - it is a super little creature, magenta and black.
Quite right Mike, I was being somewhat too prescriptive in my id...
It could well be Senecio erucifolius - the hoary ragwort - acutely pointed terminal leaflet and greyish underside to the leaves, but was it distinctly downy? The overall scruffy look of the flowers and leaf character still makes me err on the side of S. jacobaea... Either way it is a Ragwort! (Whatever they are called now...! ;-)