A Sharp-tailed Bee, but which one?

I have a regular visitor to my nesting leaf-cutter bees at present, a sharp tailed bee genus Coelioxys.  I think its either C. inermis or C. elongata.  I'm afraid without killing it I'm not in a position to confirm the I'd.  However a lovely bee and even though it has the job of a cleptoparasite of the leaf-cutters is a welcome and uncommon site here in Nottinghamshire.  Any hymenopteran experts reading this, if you can identify from the photographs then please let me know.