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John - check the mid femora for small pv bristles. If they're absent, I would say this is Hydrellia maura (though the genitalia are hardly like flavicornis). Don't worry too much about the low lunule - normally it would appear to be much higher, but it does vary a bit. H. maura is known to mine Potamogeton. If you are still concerned, I'd be happy to have a look at the male for you.

Sorry - wrote this before seeing the genitalia pics. Will look at them closely now.