Growing in water at the edge of a lake, among other wild plants.
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This species can have two types of leaves, either like here on the picture or those you describe, floating. The distinctive feature of the terrestrial leaves (as in this picture) are the adpressed hairs.
The normal aquatic plant and the terrestrial form are quite different in appearance, but are the same species. I did have a good online reference for this, but can't find it...sorry!
My Flickr photos...
Lat/Lng: 52.6401, -0.4524
OS grid ref: TF048058