The subject matter - African superstition - could have been quite interesting, especially as it does seem to have involved murder in the UK.
Also, the idea of a female police diver is unusual, as is the Bristol setting.
So how did the book end up being this boring. I don't know why, really, but I suspect it's the fact that the central characters seemed like cardboard cut-outs. Flea and Caffrey had what might be deemed to be interesting back-stories, but there was nothing interesting about them in the book.
And there is a very fine line between 'interesting' and downright unbelievable; the backstories about Flea's dead parents and Caffrey's dead brother, and especially the one about the Walking Man really were horribly unconvincing, very obviously just decoration.
As for the African superstition - she seemed to handle that quite well, not shying away from being a bit politically incorrect for some people's tastes, no doubt.
But all in all, I can't imagine I would read another book about these people.