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What the seperating feture I need to look for?
You would need to look under a microscope at the differences in the genitalia. The Trichoptera Scandinavica website is a useful resource for images - it shows Scandinavian species but we have many of the same.
The two British species of Halesus are radiatus and digitatus can be viewed on the website here:
Lat/Lng: 50.3637, -4.7186
OS grid ref: SX067550