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I think this is C. violaceus. C. problematicus has slightly striate elytra. I think the roughness is an artefact of the lighting conditions.
The two violet ground beetles can be quite hard to tell apart, especially from photos, but I'm inclined to think this is C. violaceus too. As well as the points that Richard describes, the shape of the elytra (covering the abdomen) and the pronotum (covering the thorax) also seem a better match for violaceus - I would expect the sides of the pronotum to be more strongly raised in problematicus.
See for comparison C. violaceus:
and C. problematicus:
Entomologist and biological recorder
Thanks, guys. Still learning! I had meant to make it an 'It is likely ...', but once you hit that button, you are stuck!
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
Lat/Lng: 51.23236, -0.3511
OS grid ref: TQ152494