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OK - I've been been back to the books and the specimens and I can put up a fight for P ambiguus:)

Luckily I caught a female P albimanus at the same time and could compare. There is a very clear size difference, but some similarities, for instance, the facial profile is very similar, as is the dusting pattern on the face. With the naked eye, the small specimen appears to have bands and the big one spots, but just to make sure I looked at the small one in the manner suggested by Stubbs ie viewed from behind, and it still looked like bands (using illuminated 10x magnification). The one other real difference that I noted was the colour of the hind femora. My P albimanus has entirely black hind femora, whereas the P ambiguus has orange basal half and black apical half. Unfortunately, Stubbs doesn't say anything about leg colour, so I don't know how significant this is. Ball and Morris in the Provisional Atlas of British Hoverflies say that 'although a very local species, it can be abundant in suitable localities.' The map shows a cluster of records around Greater London and surrounds, and according to Stubbs the most likely time to see it is late April. I can discount P discimanus because the antennae have orange patches on the underside of the third segment. I feel I can discount P sticticus because the abdomenal segments did not strike me as square or elongate.