short plant frowing in shady areas on the river bank in small clusters
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This looks like Himalayan balsam (impatiens glandulifera) to me, although it usually seems to grow quite tall. There is lots of it growing by streams and rivers here in Dorset and is considered an invasive pest by most people.
This looks like a similar plant also present on the banks which matches your description of being taller - however the few examples i saw of this particular variety looked shorter and had a white part to the flower whereas the others were more pink
Well spotted, CXW89. The spur is quite different from I. glandulifera and the flower looks more delicate.
Lat/Lng: 44.5, 2.2