Can anyone help me figure out which book this is?

[Terri]

Hi,

I'm really hoping someone can help me out because I'm struggling with finding this particular book!

I read the book a few years ago but I can't remember the name or the author. It was definitely set in England and I'm sure (but not 100%) that it was set in a city. Someone was kidnapping 2 people at a time and pitting them against each other. Whoever survived would be released. I'm 90% sure the main officer investigating was a woman and she would self harm but on an extreme level.

Really hope someone can help me narrow it down as it's driving me round the bend!lol