I was admiring the bright yellow ragwort shimmering in the sunshine.
Upon closer inspection the old flower heads seemed to have what looked like galls on them. It seems as though the gall midge, Contarinia jacobaeae might be the culprit according to this blog post here. Perhaps someone can tell me if they are something else?
Update: Thanks to @Ragwort_horses it seems most likely that this is Botanophila seneciella. It is commonly known as the ragwort seed fly, and along with the cinnabar moth and the ragwort flea beetle it is used as a biological pest control for ragwort.