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Do you know what size this one is (length in mm)? That would help distinguish it. My feeling is that it is P. melanarius as suggested by Iain, but the size would help confirm or otherwise.
P. nigrita would need to be dissected to distinguish it from P. rhaeticus.
Entomologist and biological recorder
Lat/Lng: 51.0721, -2.2546
OS grid ref: ST822303
Pitfall trap located under hedge in dry location