Been trying to sort out this dor beetle that I found around deer dung yesterday in VC55, using the RES Key (Jessop 1986). Struggled with that, trying to work out what tibial carina are, what they look like and whether mine had them. Better results using the Mike Hackston key suggested by Martin Harvey, and comparison of the mandlbles to the illustrated Beetle News newsletters. I've now convinced myself that this is indeed Geotrupes stercorarius - the Dor Beetle.