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Are you sure it is not common ragwort?
We have a lot of common ragwort in the field - it seems to flower until the first real frosts come.
You're probably right. I couldn't find common ragwort in the book. On looking a second time I find that it is just called Ragwort on p104 (g).
'Common ... on waste ground'. Flowers June to October. My sample still in flower today.
Lat/Lng: 53.40745330838, -2.3355066776276
OS grid ref: SJ777901