Right, Moondroog. Let's see if we can answer some of your questions.
1. hair: not Sinead O'Connor. Straight, and brushed back severely in a style that was not, as I believe, beloved of the Victorians (except possibly Jane Eyre, and she was a weirdo anyway).
2. uniform: definitely black. Short-skirted: Lewis was writing in the days before the miniskirt, so I don't thisnk he mean around the navel.
More likely the WW2 women-in-armed-forces type of skirt.
3. The cheroot: NOT, I think, hanging out of the side of the mouth, or as I think Lewis wites somewhere "seccotined to the lower lip". I imagine more at the sort of agggressive, challenging angle that Stanley portrays so well.
4. sharp features: well certainly firm. Nothing soft about the Fairy!
5. smudged lipstick: well, I think this means that she was making a contemptuous gesture towards fashion without taking any care as to how she put it on. As I've already mentioned, that's one of the few things that I think Stanley gets wrong - his Fairy's lipstick is too neat.
Stern look: at times, yes, in spades. "Impish" smile: probably more like "wolfish".
6. Eye colour: who knows? Perhaps, after a heavy night, bloodhsot?
7. age: well, she has to strike Mark as being "sexual", so given that he is a fairly stupid young man, I'd say nto much over 40, if that. More likely mid-30s.
Also, I don't really see anything "ethereal" about her - that was just Stanley's view of the WAIPs, whom I see more as the sort of fluffy blondes out of the joke book (Q: how do you know that your secretary is really a blonde? A: when you see Tippex on the PC screen. And please don't analyze that one!!) - or perhaps more like a Barbara Cartland heroine - terribly pretty, and like unto the proverbial three short planks.