This is a small (3 or 4 mm) black sawfly with pale yellow legs.  Its larvae mine the leaves of Agrimony (Agrimonia), Cinquefoils (Potentilla) and Brambles (Rubus).

The mine is a full depth transparent blotch without a clear preceding corridor, quickly and strongly widening from the start. Frass in sausage-shaped granules.

The larva has a series of black dots on the underside of the thorax

Several species of Agromyzidae flies also mine Potentilla and are present in VC55. The Nepticulidae moths that mine Potentilla and related species (see British leafminers website) have not yet been recorded in VC55

Adult Leafmine

Make sure your leaf mine photograph is backlit so that the details of the frass and larval markings can be seen inside the mine.  

Backlit photos showing the larva inside the mine

Areas where the larval foodplants occur.

Larval mines can be seen in summer and autumn when the host plants are in leaf.

Status is difficult to determine due to lack of records but it seems to be quite widespread, though perhaps infrequent.

Rare or under recorded in Leicestershire and Rutland.  The Broughton Astley record of 28th September 2018 may be the first for VC55.

http://www.leafmines.co.uk/html/Hymenoptera/F.nigrita.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/symphyta/tenthredinoidea/tenthredinidae/heterarthrinae/fenella/fenella-nigrita/

https://www.sawflies.org.uk/fenella-nigrita/