In these difficult times of Council cut backs and Government purse tightening it is reasonable to assume that the police service may not be able to meet your requirements regarding the collection or gathering of evidence.
This should not in any way distract from the excellent service the police provide, but local policing priorities may not always fall in your favour.
In general, we are talking about situations involving fraud, nuisance behaviour, employee theft, employee malingering, malpractice, false injury claims, harassment and so on.