Ragwort is a poisonous plant that is often found in heavily grazed areas, such as the rabbit warren near the old canal:
This is possibly Oxford Ragwort, a variety that 'escaped' from the Oxford Botanic Gardens (http://www.botanic-garden.ox.ac.uk) in the 1700's and spread using the railways that were being constructed at the time.
Due to it's poisonous nature, it is a favourite of the cinnabar moth caterpillar:
Who use it's toxicity as a natural defence against predators.
Interesting (and well researched) information regarding the myths associated with Ragworth can be found at: http://www.ragwortfacts.com/ragwort-myths.html - thanks Kate