Chloropidae: Cetema cf elongata

John and Barbara Ismay have produced a key to males, but looking at it, it seems to rely on characters which are not readily visible in your photo. 

Thanks Tony, looks like I will need to try and get a specimen then.

Is that key available online? I have tried to contact John and Barbara a couple of times via the Chloropid Study Group page (https://www.dipterists.org.uk/chloropid-group/home) but got no response. I'm not sure whether everything is properly set up yet.