This is a fascinating beetle (to me anyway!) it's a member of the Silphidae or burying beetles this is Silpha atrata, it is a specialised snail hunter, having a very narrow head and being able to inject poison to kill the snail, there is some information here http://www.thewcg.org.uk/silphidae/0238G.htm,
Thanks for that. Stanmore Common can now be added to the list of locations because that's where I found it along with a Violet Ground Beetle. All I have to do now is identify the spiders and harvestman caught. Many thanks again.