Dock, Rumex crispus, appears in the garden in large spreading groups, and is characterised by coarse, battered leaves. It spreads via seed produced by tiny brownish-green flowers, which appear in abundance in summer and are loved by seed-eating birds. While gardeners are less fond of this plant, it is useful to soothe the sting of a nettle when rubbled on the skin.
A tatty-looking, coarse-leaved plant that has deep, branching taproots, which will keep regrowing after being hoed out or lightly dug up.
Keep digging them out. It isn't an easy job, as you need to get every last bit - any remaining fragments of root will re-sprout. Use a fork to avoid breaking up the root. Regular close mowing will keep most lawn infestations in check, as it prevents them from flowering and setting seed.
Use a total weedkiller, such as glyphosate, spraying directly on to the plant. Avoid spraying on a windy day and near other desirable garden plants.