We finally have an answer. As she revved up for her evening scream, I noticed a little something sitting on her gum behind her tooth. A quick* explore revealed sharp pointy things. Oh yes, she's finally doing something right on target, and is gaining her first six year molar. Marvellous. Why it should only bother her in the evenings I'm not entirely sure, but I'll take that over constant distress. It explains the nappy rash, it probably explains the atypical lurgy she's had, it explains why heavy duty painkillers are working better than sedatives to get her off to sleep.
I really hope that now the point is through the rest follows quickly and less painfully. As a baby, she timed her teething well and we had a week or more of misery for each pair of new teeth. First two, Mothering Sunday. Second pair, our summer holiday in France. And so on. That was a week of solid misery, at a time when she had finally started outgrowing the constant crying. Considering that she's confining the misery to a few hours each evening, does this mean the whole process is going to be stretched out for weeks on end? Jolly well better not be!
*and, may I say, extremely brave - Mog has an overactive bite reflex, you don't want to get your finger stuck in that.