I'm not sure about that Portevinia maculata, James. Females are less distinctively marked than males but they still have greyish spots, and clear wings, also orange antennae. So I wonder if it might be a Pipiza species? That would fit with the slightly darkened wings.
Are you in the right area for Chrysotoxum verralli?
These are tricky but close examination of the antennae can be a help. C. cautum has the first segment shorter than the second. With verralli the first and second segments are equal in length. Verralli is also a bit smaller in size.