In mud at edge of pond
No interactions present.
I agree this is not B. cernua and could be tripartita var intergra, but these days you need to consider B. connata which has warty achenes as opposed to smooth ones. It is, however, usually by canals.
It seemed too small to be a Bidens (only about 50mm tall). I will try to get back a couple of times over the next few weeks to see if it grows any taller and to check the achenes.
Entry edited 06/02/2013 to add a photo of the achenes (achenes collected late Oct/early Nov 2012 and photographed on 02/02/2013).
Achenes appear to support ID as Bidens tripartita - more or less flat with 2-3 bristles, tapered from apex to base, backward pointing barbs on both the achene and the bristles.
Lat/Lng: 51.6145, 0.0166
OS grid ref: TQ397925