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I have now another name to propose, according to a French foro: Aster salignus
... is (IIRC) the commonest of the North American asters in Britain. The key characters given in Stace are
A. x salignus: Outer phyllaries widest below middle, <= 0.7mm wide, rather neatly appressed to capitulum.
A. novi-belgii: Outer phyllaries widest at or above middle, > 0.7mm wide, with conspicuous leafy distal 1/2 loosely or unevenly appressed to capitulum.
Lat/Lng: 42.95, -3
Spanish Basque country