My understanding of the ending (Spoilers)
Ok, so Kenneth Monk had a sexual relationship with his step-sister and went on to murder her, with some help from his mother.
So am I right in thinking that the ghost that was trying to communicate with Laura and Guy was NOT their daughter Kimberley, but Kenneth Monk's step-sister? And the ghost was making Guy and Laura think that the messages were coming from their daughter, so that they would kill Monk.
But Monk didn't kill Kimberley, the original suspect Dennis Law was actually guilty. That was how I interpereted the ending anyway. I figured that was the reason why they showed Law watching Kimberley singing at the end.
Good film anyway, clever and unpredictable.