I'd like to suggest a re-organisation of your photos. The second and third photos show Callitriche stagnalis, but the first one (the main photo) is confusing: there is a bit of the stem of Callitriche on the left but the photo emphasises some ovate leaves, growing from the same spot as the Callitriche stem. These ovate leaves are very unlike any Callitriche I am familiar with, and it is likely another species.
The second photo shows the habit of Callitriche quite nicely (and the little unknown seedling is not featured), as does the third photo. I suggest you make the second photo your main photo and trim out the first one altogether. I'd be happy to confirm the sighting as Callitriche stagnalis .