State immunity — Jurisdictional immunity — Employment — Action challenging termination of employment — Applicant employed as bilingual secretary in diplomatic mission in London — Whether solicitor's letter directed to employer regarding status of staff in English law and circulated to staff prior to conclusion of contract of employment constitutes a “prior written agreement” to submit to jurisdiction — Section 2(2) of the State Immunity Act 1978 — European Convention on State Immunity, 1972 — Whether immunity from proceedings concerning employment of members of a mission applies in case of secretary whose appointment has not been notified to foreign ministry — Section 16(1) (a) of the State Immunity Act 1978 — Vienna Convention on Diplomatic Relations, 1961 — The law of England