Section 78 of the Police and Criminal Evidence Act gives the court powers to exclude evidence from a trial which has been unfairly obtained.
It does not state that the courts must exclude evidence, only that they can. In fact the legislation, and case law specifically allows for evidence that has been obtained improperly to be admitted.
When a court decides if evidence is admissible or not, it must look at the value of that evidence. For example, if a police office did not read the caution correctly to a murder suspect during his arrest and the murderer said, “I killer her and the knife is over there”, and police then recovered the knife, the court is going to allow the evidence of the knife, and probably the confession. If, at the other end of the spectrum, the police used an illegal surveillance to prove evidence of driving without insurance, that evidence may be excluded.
This section has resulted in numerous case law, and is not to be confused with Section 76, which deals with confessions obtained through oppression.
This article is an attempt to analyse the principles which the courts use to decide whether or not to exercise the discretion to exclude. It starts by examining the decisions of the appellate courts in order to try to identify the factors which the judges regard as relevant to this discretion. Secondly, it examines the possible policies which might underlie a discretion to exclude. It then attempts to match the practice to the policies, and concludes that ‘fairness as fair play’ is the dominant policy currently being used. The conclusion considers how the changes in the rights of the suspect (in particular the right to remain silent) introduced by the Criminal Justice and Public Order Act 1994 are likely to impact on the operation of s 78. Source