In R (Freedom and Justice Party and others) v Secretary of State for Foreign and Commonwealth Affairs and the DPP  EWHC 2010 (Admin) the High Court (Administrative Court) was faced with this question of law:
"Whether members of special missions visiting the United Kingdom with the approval of the First Defendant ("the FCO") enjoy personal inviolability and/or immunity from criminal process pursuant to a rule of customary international law to which effect is given by the common law."
The court (Lloyd Jones LJ and Mr Justice Jay) held:
(1) Customary international law requires a receiving State to secure, for the duration of a special mission, personal inviolability and immunity from criminal jurisdiction for the members of the mission accepted as such by the receiving State.
(2) This rule of customary international law is given effect by the common law.
This interesting judgment is available via Bailii -here