I had never even heard of this plant so it is quite thrilling to find out its proper name. It is found all over the British Isles but prefers warmer conditions. The flowers produce abundant seeds which can float and disperse in water which is why it is common near rivers.
It has been used in herbal medicine as it contains dicoumarol (an anti-coagulant). It is useful for honey production and for its nitrogen fixing ability in preparing agricultural soil for future crops.
I used the website of The British Botanical Society (link opposite) to identify this plant. A really useful website.