If you're by a river bank or in a fen, keep a look out for stinging nettles with particularly long, narrow leaves. And then if you're feeling brave, try brushing your hand against them. The chances are you won't be stung, as this leaf shape is very characteristic of Urtica dioica subsp. galeopsifolia, the stingless nettle.
The classic locality for this subspecies is Wicken Fen, but it occurs more widely in damp habitats. The images above were taken on the bank of the River Nene at The Boardwalks LNR, where it occurs alongside the normal painful nettle, as well as with some intermediates.
If you examine the leaves closely you'll find that there are scarcely any stinging hairs, but instead there is an indumentum of dense simple hairs. The lowest flowering branches are at nodes 13-22 and it flowers from mid-July onwards, about a month later than common nettle.