Keith Dodd writes:

A friend of mine, Peter Hill, pointed out this plant to me.

It is uncommon, and there are two small patches about half way along the riverside path within the reserve.

It is called Bistort or Easter-ledges. My book tells me it is edible, and is eaten in Yorkshire and the Lake District as Ledger Pudding (chopped plants with hard boiled egg), though it's most common in Lancashire.

Could this be the origin of the well known gravy browning, Bisto?