- Beetles (weevils)
The list of options for that foodplant is very small (one micromoth and one fly), and this looks like a fly mine, so I'd have to opt for Cheilosia semifasciata - which is a hoverfly.
However (and this is where it gets very interesting), the other Cheilosia sp. aren't leaf-miners. Information about Cheilosia semifasciata is scant - there is no image of the adult in the standard reference - but it appears to be an oligophage: it mines plants that belong to a single family (another host plant is Orpine - Sedum telephium). So it appears that one member of a hoverfly family has independently discovered leaf-mining, and on a plant that is not mined by any other fly. Interesting.
Edit: it turns out that this hoverfly is extraordinarily rare, and I'll be monitoring the situation over the next week or so.