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Tim, I did wonder about the hybrid. Is it just that it has leaves like M. aquatica and flowers like M. arvensis?
hasn't enough "leaf" in the inflorescence for arvensis, and too many whorls for aquatica and this looks more hybrid than either - but there are others like smithiana which can look the similar. Hard from photos without fertility and stamens and being able to see the calyces
Lat/Lng: 54.2, -2.0
OS grid ref: SE0188