Many of the plants appeared by the riverside after a major re-development of an industrial area. Not sure what it was, but had a guess
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Could be Fat-hen, Chenopodium album. The plant looks too compact overall to be Common Orache. Leaf shape is also more Fat-hen like, oval to diamond shaped. I would expect thre orache to be more lanceolate towards the upper plant.
Hope this helps.
I think one can just see the structure of the flowers on this photograph and they look hermaphrodite, which means it cannot, apart from anything else, be an Atriplex as they all have unisexual flowers (and there are none of the large triangular bracts which characterise Atriplex). I agree with Bill that C. album is the most likely, but Chenopodium species are seriously dfficult to ID to species as there are so many of them , although most are rather uncommon aliens. For many of these you need ripe seed and a good microscope. Leaf shape is pretty plastic, but there are some reasonable illustrations in the Plant Crib, including a section on telling vegetative C. album from A. patula!
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Lat/Lng: 51.215329140018, -1.4795612094167
OS grid ref: SU364463