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The fly stayed completely still during the three minutes that I observed it; it eventually flew away. The images suggest to me that this was a female laying eggs,(and as she was so small) anchoring herself on the leaf surface with her antennae to avoid being blown off.
For "antennae" substitute "front legs" (Image 2); Chironomid antennae not likely to be that long.
Lat/Lng: 51.4337, -2.7624
OS grid ref: ST470707