The Extradition (Drug Trafficking) Order 1997

18.  A witness, expert or other person who consents to give evidence in a proceeding or to assist in an investigation, prosecution or judicial proceeding in the territory of the requesting Party, shall not be prosecuted, detained, punished or subjected to any other restriction of his personal liberty in that territory in respect of acts, omissions or convictions prior to his departure from the territory of the requested Party. Such safe conduct shall cease when the witness, expert or other person having had, for a period of fifteen consecutive days, or for any period agreed upon by the Parties, from the date on which he has been officially informed that his presence is no longer required by the judicial authorities, an opportunity of leaving, has nevertheless remained voluntarily in the territory or, having left it, has returned of his own free will.