Found growing on 2 deep puddles in Birchwood Nature Reserve, Lincoln. I am thinking not Duckweed, down to the pointed leaves.
No interactions present.
A list of British Callitriche species with links to named images:
An iSpot Project- HELP WITH IDS - UK & Ireland Community - PLANTS
The treatment in Stace (3rd edn) has 7 species. It also says "difficult to identify certainly without a strong lens or microscope".
Lat/Lng: 53.2199, -0.6116
OS grid ref: SK927701