The Elements of the Offence
It is a common law misdemeanour to pervert the course of public justice. The offence is committed where a person or persons:
(a) acts or embarks upon a course of conduct,
(b) which has a tendency to, and
(c) is intended to pervert,
(d) the course of public justice: R. v. Vreones, ante.
A positive act is required. Inaction, for example, failing to respond to a summons, is insufficient to constitute the offence. An attempt or incitement or conspiracy to pervert the course of justice is likewise indictable.
Conduct Capable of Amounting to the Offence
Any act or a course of conduct tending and intended to interfere with the course of public justice will amount to the offence. Such acts may also be contrary to specific statutory provisions or amount to contempt of court.
Obstructing the police
Deliberate and intentional action taken with a view to frustrating statutory procedures required of the police can amount to the offence.: R. v. Britton  R.T.R. 502, CA. The case is a common law example of obstructing the police in the execution of their duty contrary to the Police Act 1996, s.89(2): see Dibble v. Ingleton  1 Q.B. 480, DC.
Where persons conspire to obstruct the police and, by so doing, pervert the course of justice, the conspiracy is different in kind from a simple conspiracy to obstruct the police, being more reprehensible, and requiring more severe punishment.