I was not impressed. I hated that the narrator was unreliable. Instead of thinking that it was a good twist when she admits that her "letter" to Tess has simply been her thoughts while she lies on the floor of the bathroom building, I found it annoying that Lupton tried to pull a "gotcha".
I thought putting the narrator into the present moment did not increase the suspense (as it was meant), but instead made me distrust everything that Bee had presented. I also found it ridiculous that William would have had sex with Bee just prior to bringing her to the abandoned toilets. Why would he want his sperm to be found inside of her? Wouldn't that make it harder for the cops to believe it was another suicide? And, what are these toilets doing unlocked anyway? I've been to Hyde Park (lived 5 blocks from there and walked through the Lancaster Gate myself on an almost daily basis for several months) and there are lots of random buildings around, but they are not left unlocked. Even if one had been left unlocked originally (and William got lucky when he murdered Tess), they wouldn't be unlocked the second time (security should/would have clamped down).
"A child's body is so much a part of who they are, maybe because we can hold a little boy in our arms. We can hold the whole of him. But when we grow too large to be held, our body no longer defines us." WTF? I think our adult bodies are just as defining as our child bodies. What about sex? What about stereotypes and quick judgements? I get that Beatrice (when she says the above) is trying to distance herself from Tess's body and is commenting on why the post mortem did not upset her, but that is not because Tess is an adult (in fact, up to this point Bee has mostly treated Tess as a child), but instead is because she believes in Tess's soul and personality as a separate entity from her physical body.
Twice Lupton refers to "flying back past the second star to the right and on until morning" (yep Peter Pan) and once she mentions her inability to "sail back through the weeks and in and out of the days" (yep, Where the Wild Things Are). I found both of these references banal and unnecessary. This is not a Peter Pan story (yes, Leo does not ever grow up but that's because he is dead); it is supposed to be a medical mystery.
I did like "I wasn't the slickly presented, self-controlled person I'd been in the States, and I wondered if that encouraged other people to let the untidy aspects of themselves and their lives show in return." I liked that Bee was so aware of her self and relation to others (and the social psychological implications of how others will react based on how she self presents).
Yes, it is a page turner, but nothing much there and the unreliable narrator in the end just turns me off.