This plant has now been identified as Charlock!
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Thanks for your id but I don't think its a willow - it almost looked like a type of mint at first glance (thickish leaves and stem) - now looking at the photos it almost looks like a Viburnum! The flattish, yellow flower buds would fit with V. lantana, I suppose. Still not sure - will have to revisit the site!
The more I look at the photos I took, the more befuddled I am! Your willow suggestion does make me think its some kind of submerged shrub, but what? Baffling...
The flower buds can't really be seen from the photo above, but they are yellow and clustered in what seems will be a flattish flowerhead - perhaps umbel-like when open...
Lat/Lng: 52.8154, 1.4274
OS grid ref: TG310297