One article that interested me was "Ambiguity in recording Centaurea (knapweeds) taxa using MapMate". This had coloured pictures of the phyllaries of what are considered to be the three British species (C. nigra var. nigra, C. nigra var. nemoralis and C. jacea) though it pointed out that hybrids were common. ('Phyllaries' are the small leafy structures, or bracts, that surround the base of the flower/seed head.)
I immediately pulled on my boots and set off to my favorite patch of 'old' meadow where I found plenty of knapweed seed heads still perched on stiff, dead stems above the fallen grass. These were plainly the nominate subspecies Centaurea nigra var. nigra.