Looks a lot like Duck Weed but the leaves are a bit more oblong and have clear veins ??

UPDATE July 3rd
Peter Nicholson has suggested it may be a 'starwort' here is an image of the [common water starwort] that is similar to the above.
https://www.naturescape.co.uk/product/common-water-starwort/

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Original Post

A new discovery. Whilst showing Pete Nicholson around as he was looking for site to establish Triangular club rush he pointed out a mass of 'Newzealnd Pygmyweed'' by the damaged drain in the gully going north.

Will take a picture and a sample when I next go to check on the adjacent Japanese Knotweed. ✅

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Meanwhile here are links to some info: [ New Zealand Pygmyweed     Link opens new tab/window to (canalrivertrust.org.uk) https://ywt-data.org/inns-mapper/species/new-zealand-pigmyweed https://blog.invasive-species.org/2023/05/17/crassula-the-potent-invasive-plant-being-challenged-by-a-microscopic-mite/ Problem: Under Schedule 9 of the UK Wildlife and Countryside Act 1981