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Many thanks Danny.
All the features fit, except one, the abdomen on my specimen has grey 'blobs' on each tergite whereas the specimen in 'Stubbs & Drake' is all black. The description also states a black abdomen. Could mine be recently emerged and the grey is just an artefact of being a 'fresh' specimen?
Cheers,
Steve.
Edit: just looked at some pics in Malcolm Storeys excellent Bioimages website and it is clear that these grey markings are only visible in lateral views - from a dorsal position the abdomen appears entirely black; mystery solved and thanks again Danny:)