Confidence: I'm as sure as I can be.
Notes: Broad tear-shaped forewing held flat. Yellow margin along leading edge is pale and does not continue around the corner at the end of the wing. Common Footman and Scarce Footman both have more conspicuous yellow margin that continues around the bottom edge of the wing. Scarce Footman also holds its wings tightly wrapped around its body.
I have not seen it reported in any guides but I have the impression that Dingy Footman may also be distinguishable from the other species mentioned by leg colour. It has very grey legs but the Common and Scarce have yellowish legs - at least the third pair. This is just an impression I have and I would be interested if anyone else can corroborate this and comment on whether or not it is a reliable character.