Hello
There is a very comprehensive website covering hoverfly data. This is the Hoverfly Recording Scheme website at
Hoverfly Recording Scheme - Index
There is a valid map for all species of Chrysotoxum. At the moment two species are likely to occur in Devon: C. elegans and C. cautum - but C. elegans is the more likely. I would not expect C. verrallii that far west. Curiously that species has not moved at all over recent years. In fact we think it is possibly declining.
My instinct for your photo says C. elegans but I am cautious - can you give some data please - that may help matters.
Regards
Roger Morris
Hoverfly Recording Scheme