Sovereign immunity — Foreign State — Restrictive theory of immunity — Contract of employment — Employment on staff of diplomatic mission — Employment in clerical capacity — Employee a national of the employing State — State Immunity Act 1978 — Whether applicable to contract concluded before Act entered into force — Sovereign immunity at common law — Nature of contract — Whether employment at diplomatic mission commercial — Whether employee involved in public acts of State — Nature of breach of contract — Whether relevant under restrictive theory — International law and English law — Value of foreign decisions — European Convention on State Immunity, 1972 — Whether reliable evidence of customary international law — The law of England