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There is an OPAL guide to duckweeds at http://www.opalexplorenature.org/sites/default/files/7/file/Duckweed-gui....
A quick glance suggests to me it is common due to bright green colour, but I haven't got any scale to go by.
This looks a bit "humpy" which might suggest L. gibba. We need to see the underside.
Lat/Lng: 52.1277, -2.3261
OS grid ref: SO777477