Thank you all very much. I've checked my diary and I did buy some centaurea montana in Oct 2017, and I also grew Hesperis (purple and white versions) last year. Given the size of it, perhaps it is the centaurea. Love the way people here help the less informed. Thanks again.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Centaurea [ perennial cornflower ], i think.
When you don't even know who's in the team
S.Yorkshire/Derbyshire border