I didn’t realise that there were so many types of Forget-Me-Nots. The best fit was the Water Forget-Me-Not, which obviously likes damp places, water margins. It has purple-tinted sheathed buds and long stems with branched buds and long oval leaves. It can grow up to 30cm, though I think it grows more mat-like in the water. This one is in a low-lying reliably damp part of the watermeadow.