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Indeed - I did some lunchtime reading, so I knew what I was in for later. I figured I could discount some because their flight times were spring or early summer, and this specimen is from August. Also a couple had very strong associations with very particular habitats (sand dunes, fens) and this one came from an area of deciduous woodland with sunny glades and was found on an umbellifer.

The results of the headscratching session is that I fell at the first hurdle, being unable to distinguish for certain how many post sutural dorsocentral bristles it had - some were missing, but some I think must just not be very strong. I worked the key through with both choices to see what I got, and it only worked if I opted for 4 psdc, which looking at the photos again is probably right. The other important characters that I noted seem to be:

a) haustellum shorter than length of head
b) 3rd antennal segment narrower than front femur
c) tegula much the same colour as basicosta - if anything slightly lighter.
d) 2 dorsal marginals on tergite 2
e) 8 hairlets below frontal bristles

Result: S geniculata - apparently by far the commonest species.

I checked out a few of the others to try to make sure I wasn't just giving myself the answer I expected. For instance, it is not S cristata, the next most likely candidate, as S cristata does not have dorsal marginals on tergite 2.

The expression 'pops out' is not quite how I would describe the process. Dragged bit by bit from a jar of treacle is more like it.