Some elements that I remember:
- Bluey-green cover with a medallion on it
- Almost certainly set in London
- Half of [city] is semi-abandoned, maybe partially flooded?
- Begins with protagonist tracking/hunting an imp or jackal-like that has been stealing children
- Protagonist is navigating/tracking by placing milk in a bowl of blood
- At some point, protagonist (and friends) stumble upon and rescue a girl who is essentially feral
- I remember one of the type of baddies chasing her were described as things from the ocean, and they made the air cold, clammy and salty when they arrived in the night
I got this book from the library when I was young - which would be around 10 years ago, give or take - and forgot the name and author, but the feeling of it stuck with me and I have been struggling to identify it ever since.
Any pointers - even just how I might better search for it myself - would be greatly appreciated!