There are 3 species of Cheilosia with a female that has a yellow tip to scutellum - longula/scutellata/soror.
soror hasn't made it to Wales yet and is only found in Southern England. longula has a swollen hind metatarsus, although the photo seems to hint at this, it is not swollen enough to fit longula.
Therefore this is Cheilosia scutellata, a good pointer that this is scutellata is the front and middle tarsus are pale and is diagnostic feature of the species. [record confirmed by HRS]