Sensual overtones or not, JJ was shown to be OVER. Nat showed no further attraction to Jane...and it could just as easily have been based on looks as well! Nat calls her 'beautiful' as well, but makes no remarks that compliment her character or personality. In addition, he mentions that he was 'eager to continue' the discussion with Kitty; Nat enjoyed talking with her, whereas Jane was really a rival, NOT a friend in any way; she turned on him in an instant! Before he foudn out Kitty was alive, Nat was depressed throughout most of PG because he was friendless and tired of politics. I don't think he would look much for a partner in that world, least of all someone who wouldn't stick by him.
Kitty showed that she would stay with him to the end, and is more than a voice of reason between Nat and Bart. She may be better at masking her feelings, but she obviously cared about both Nat and Bart enough to be worried about them when Nat got hurt. And the 'beautiful' remark was just sort of a realization on his part; he was definitely attracted to her BEFORE then.
MUSICA DIABOLOS
http://www.fanfiction.net/u/962826/
Nathaniel + Kitty Forever!
"It's two thousand, one hundred and twenty-nine years since Ptolemy died. I still wear his form. Who do you think's the lucky one?"
- Bartimaeus, Ptolemy's Gate