Confidence: I'm as sure as I can be.
Notes: The right hand photograph, viewed at maximum size, shows the outer phyllaries to have their undivided portion long and lanceolate in shape (not broadly ovate as in true C. nigra). The stems are only slightly swollen below the flowerheads. Radiate (technically "pseudoradiate") flowerheads are common in this species and rare in true C. nigra. The lower leaves should be more lobed, but these are not visible in the photograph and it's an uncertain character anyway. This is the plant that back in my youth was regarded as a separate species, as "C. nemoralis", which was then downgraded to a subspecies of C. nigra, and then largely dismissed altogether. But in vol. 4 of Sell & Murrel it re-emerged as a species again, though with change of name, and it is also recognised as such now in Stace ed. 3 (taken by the BSBI as the standard for identification). It is common in the south, especially on light and chalky soils. [Since the ID above has been agreed by another with an expert badge, the way iSpot works, I will be outvoted on this and this will not be shown as the "likely ID". But on current concepts, I am right!]