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Recent work by Peter Sell has shown that there are several, reasonably distinct entities within the Polygonum aviculare group, not covered by current books. The account has not yet been published (though is used in Arthur Chater's Flora of Cardiganshire). I have my own interest in these plants and believe Sell is correct and that the revised classification will be workable (but may not cover all taxa in Britain).
Anyhow, to get to the point, the photograph looks very like P. agrestinum, a narrow-leaved, upright species of agricultural land and disturbed ground. I have known this for years and been baffled by it, as it does not match either P. aviculare in the strict sense, nor "P. arenastrum". When I saw the draft new account and photocopies of the taxa (courtesy of Arthur Chater), it was like recognising an old friend! It is probably common, certainly so in southern Scotland and in Cardiganshire, no doubt the same elsewhere.
It would be premature to use this name as a formal revision, so I have added agreement to "Polygonum aviculare agg.".
Lat/Lng: 50.8757, -0.0165
OS grid ref: TQ396103