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... the dark blotch on the leaves characteristic of Persicaria maculosa (and Persicaria amphibia), so it might be Persicaria lapathifolia. On the other hand I can't see the glands on the peduncle which are said to be frequent on this, and absent to sparse with Persicaria maculosa. The other characters that distinguish these two species need a hand lens.
The dark blotch on the leaves of Persicaria maculosa varies from prominent to scarcely visible.
Lat/Lng: 54.710322, -1.685774
OS grid ref: NZ203350