Can someone enlighten me as to what this is please?
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It appears to be water mint from the small rounded leaves.
I presume you also checked by bruising one old leaf. Whilst it smells of mint it is a much milder aroma than normal garden or spear mints.
crush a leaf and it was minty, I then failed to find water mint in the index of my book (great observation skills!) so I wasted far too much of my time and now yours.
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Lat/Lng: 51.9552, -0.4201
OS grid ref: TL086297
Growing in chalk stream