Came to a garden light trap
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It looks good for Stenophylax permistus, but the species in genus Micropterna are rather similar. There is a difference in size: permistus is 18-22mm wing length, Micropterna are 13-18mm, so I'm not sure how confident we can be of where your "close to 20mm" estimate lies!
Entomologist and biological recorder
Thanks. Regarding the size, it was large, so probably 18-22mm, but I'm clearly not accurate enough for a verified record. I'll know to take the measurement next time I catch one in the light trap!
Lat/Lng: 51.5, -1.2
OS grid ref: SU5886