This is the largest genus in the family Eulophidae with nearly 1,000 species described worldwide and over 100 UK species. Very difficult to identify to species level. 

Aprostocetus: Submarginal vein in forewing with 2-5 (usually 3 or more) dorsal setae, propodeum with an unbranched medial carina (ridge).
Tetrastichus: Submarginal vein in forewing with one dorsal seta, propodeum with a carina (ridge) branched like an inverted "Y".
Quadrastichus and Oomyzus also share the single seta on the submarginal vein with Tetrastichus, but they lack the Y-shaped propodeal carina. 

Unless identified by a recognized expert, explain how the specimen was identified and include photos of key features. 

Parasitoids with a very wide range of host insects. 

Common.

Unknown.

https://keys.lucidcentral.org/keys/v3/eulophidae_parasitoids/