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Caught out again by the System! Stace considers Poterium to be valid.
The Plant List says Poterium sanguisorba L. is a synonym of Sanguisorba minor Scop.
Poterium sanguisorbia L. and Sanguisorba minor Scop. are (nomenclaturally) valid (and legitimate); which is (taxonomically) correct is to a degree a matter of opinion.
One study found that uniting Poterium and Sanguisorba produces a genus which is paraphyletic with respect to Acaena (and a few other genera). If this result holds up, and if one subscribes, as most people do nowadays, to the principle that taxa should be monophyletic, you either have to separate these two genera, or sink all of Sanguisorbinae into Sanguisorba.
Lat/Lng: 50.842386666667, -0.061353333333333
OS grid ref: TQ365065