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By distribution this could be either the Black or Violet Oil Beetle, and I don't think an ID to species can be ascertained as the rear edge of the thorax is obscured in both images.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
Whilst I 100% agree with your diagnosis (one of two and not possible to be sure which), I am a bit confused by the distribution thing... There are four oil beetle species down here in Devon.
Lat/Lng: 50.6669, -3.2707
OS grid ref: SY102860