“… on the facts of the present case, it must follow from a finding that there was an intention to impede police investigations that there was an intention to pervert the course of public justice. On these facts, there can be no explanation for intentionally impeding police investigations other than an intention to pervert the course of public justice. … the jury clearly rejected the appellants' explanations as to why they disposed of the shotgun and cartridges and concluded that the appellants intended to impede police investigations. … It was in the sense that what impedes police investigations will usually also have a tendency to pervert the course of public justice that we read the statement of Lord MacDermott LCJ in R v Bailey [1956] NI 15 at 26 that the administration of public justice— 'comprehends functions that nowadays belong, in practice almost exclusively, to the police, such as the investigation of offences and the arrest of suspected persons.'”