State immunity — Head of State immunity — Immunity ratione personae — Immunity ratione materiae — State Immunity Act 1978 — Section 20(1)(b) — Members of family of head of State — Extended royal family — Half-brother of King — Half-brother’s son — Whether Princes forming part of household of head of State — Whether Princes immune from proceedings pursuant to Vienna Convention on Diplomatic Relations, 1961 — Article 31(1)(c) — Princes engaging in alleged commercial activity outside United Kingdom — Whether commercial activity exception applicable — Diplomatic Privileges Act 1964

Relationship of international law and municipal law — Treaties — Interpretation — Vienna Convention on Diplomatic Relations, 1961 — Article 31(1)(c) — Diplomatic Privileges Act 1964 — Diplomatic immunity — State immunity — Functional basis — Customary international law — State Immunity Act 1978 — Section 20(1)(b) — Foreign heads of State — Immunity ratione personae — Immunity ratione materiae — Members of family forming part of household of head of State — Definition — Alleged commercial activity outside United Kingdom — Whether personal immunity applicable — The law of England