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The Small Fan-footed Wave doesnt have the conspicuous dark markings of Single-dotted Wave on the trailing edge of the forewing. SFFW has very faint markings by comparison.
That's allowed me to compare them in Townsend & Waring and yes I can see exactly what you mean.
Lat/Lng: 51.9587, 1.0215
OS grid ref: TM076332