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Am I right on this? I struggled a bit even to settle on the family. If it is Hydrellia, I have learnt from previous threads not to expect a species determination ;) These flies have been in my mother's garden in Yorkshire at least for the past few weeks. There is no standing water. They always seem to be within about 300mm of the ground, which is well covered by plants. They seem to favour Alchemilla mollis. See below for some feeding behaviour.

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Another view:

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and the face:

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I watched one of them jump on a small nematoceran which then seemed to escape. When I got the photos home I found that it was stripping it of mites then letting it go. Quite a useful valeting service:)

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Sorry about the shaky picture

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Cleaned up and ready for release:

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Never heard of this. Really interesting observation. Could never have tought of this... All the cecidomyiids left the fly alive? Do the last 2 photos show the same pair (same fly - same cecid?)

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Yes, the last three pictures are all of the same event. I only saw this happen once, and the 'victim' got up and flew away. I got the pictures because I use the camera as a field lens - my eyes are not good enough to see what's going on at this scale:|

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Looks like Hydrellia griseola - that's the most likely species to be found away from water, as the larva mines terrestrial plants - most species are breed in aquatic or emergent vegetation.
As for the feeding habit - I'll check this one out and get back to you!

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Thanks Tony, it's very good to have a species! Next time I go to my mother's I'll have a look for nearby leaf mines. Any particular plant it likes?

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I can give you a list of about fifty species! Grasses and emergent vegetation generally - the mines are difficult to spot (not as obvious as agromyzid mines).