Sir Ian Blair in this year's Dimbleby Lecture (transcript here) has said that the public must decide what they want from the Police.
What the police are for is a surprisingly difficult question. The first time I ever saw it addressed was in a a remarkable article called "Broken Windows" published in the Atlantic in 1982.
Broken Windows has cast a long shadow. It is really the inspiration behind "zero tolerance", neighbourhood policing, and all sorts of initiatives.
You can read it (in Adobe Acrobat format) at http://www.manhattan-institute.org/pdf/_atlantic_monthly-broken_windows.pdf