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Second opinion differs :)

I think Ch. intermedium. The third antenna segment is even longer than in Ch. cautum (3 > 1+2) and pattern on abdomen is typical for intermedium - yellow strips on tergites leads from the middle of front margin to the corners of hind margin and additional (rare) yellow strips are quite blured.

(Ch. cautum has also very distincintive patter on abodomen, but different). Try to study some reliable websites such as this https://www.flickr.com/photos/63075200@N07/sets/72157629614639799/

unfortunately, Ch intermedium is not a British species and thus not included in this website. See also diptera.info gallery https://diptera.info/photogallery.php?photo_id=3233