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Thanks Theo.
I may well be barking up the wrong tree (I often do).
But the key I use has only nigrifrons as a smaller size (5-7mm), and sylvatica, albiseta, aristata and fulviventris as in the right range of 7 - 9 mm.
And the key has albiseta as having a broad yellow central band below the antennae, but mine has a broad brown band. This is why I was puzzled - nothing seems to fit.
Another image uploaded of the face of the female, showing the brown band in the middle.

Nigel