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Ewww - as you have a male I would say genitalia are your best best - they are fairly characteristic if you can clean them up a bit. You need both a lateral and a dorsal/ventral view of them and compare them to the figures in our revised key. Also eye- vs. frons-width measurements will help a great deal ... all other features are a bit hard to work out on photos :)

EDIT: C.atricosta is very rare in the UK so if it is looking like that then I'd need to see the specimen myself :)