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At least I was right about the genus.
albitarsis is very, very common at the moment, especially on Creeping Buttercup in damp situations. ranunculi is less common (and not very well known) and impossible to separate without taking a specimen, so this could be either.
My Flickr photos...
having done a bit of reading since being put on the right track - though I'd have expected "ranunculi" to be the one favouring buttercups! They are prevalent in the semi-woodland area I found it in.
Lat/Lng: 54.6134, -1.3782
OS grid ref: NZ402244