The Stonebridge Allotment Society have recently highlighted the issue of Indian or HimalayanBalsam, Impatiens glandulifera, that has been found around the pond this year. This very pretty alien is very invasive and can quickly colonise river banks, sometimes to the exclusion of other plants. There is an obligation under law to control it and stop it spreading onto neighbouring land. As such it is something to look out for on the stream, so if you see it, please do let the group leaders know. The following link explains more:
Thanks to Bob Gomes and the Stonebridge Allotment Society for highlighting this issue and for the photograph of this attractive but unwanted plant.