about 12cm high and 10cm across
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I have seen leaves like this before, but cannot put a definite name to the plant. Sorry.
Revision: gardener's suggestion of water pepper can be tested by tasting a small amount of leaf - bitingly hot if he's right. Haven't seen it since Wilden Marsh in the 70's, but remember the leaves as more lanceolate.
I've just looked at the photo more closely and realised the shape of the leaves! I take back my suggestion of Water-pepper but have no idea what it might be instead.
You'll have to go back and photograph it in flower.
Lat/Lng: 51.2, -0.8
OS grid ref: SU8849