On 11 September 1973 General (later Senator) Pinochet came to power in Chile. On 11 March 1990 he resigned as head of state after a controversial time in office (see Contemporary Review, November 1998). In 1998 he came to England for medical treatment. While he was in England Spain sought to extradite him on a number of charges. The charges eventually considered by the House of Lords included conspiracy to torture (in the futherance of some of which conspiracies it was alleged that murders were committed in various countries including Italy, France, Spain and Portugal), conspiracy to take hostages, conspiracy to murder in Spain, attempted murder in Italy, and torture on various occasions. Some of the charges related to the time before Senator Pinochet became head of state, most of them to the time when he was head of state, but none to the time after he ceased to be head of state.
The issue of whether Senator Pinochet could legally be extradited came before the House of Lords, the final court of appeal in the United Kingdom. In fact it was considered there twice. The first decision of the House was famously set aside on the grounds that the Judicial Committee was not properly constituted. The speeches constituting the second decision were delivered on 24 March 1999. They can be found sub nom Regina v Bartle and the Commissioner of Police for the Metropolis ex parte Pinochet at http://www.parliament.the-stationery-office.co.uk/pa/ld/ldhome.htm and sub nom Regina v Bow Street Metropolitan Stipendiary Magistrate ex parte Pinochet Ugarte (Amnesty International and other intervening) (No. 3) at  2 All ER 97.
There were two broad issues before the House. First, whether the crimes alleged were crimes in respect of which the English law permitted extradition. Second, whether, if so, it would nonetheless be illegal to extradite Senator Pinochet because he enjoyed, as an ex-head of state, immunity.
In English law an 'extradition crime' (one in respect of which, absent a correct plea of immunity, it would be lawful to order extradition) must constitute a crime in the laws both of the country seeking extradition (in this case Spain), and of the country from which extradition is sought (in this case England). This is called the 'double criminality rule'.
The first important decision of the House was that in deciding whether a crime is an extradition crime, one has to ask whether it would have satisfied the double criminality rule under the laws of the two relevant countries at the time it is alleged to have been committed. It is not enough that, at the time that the request for extradition is made, the rule is satisfied. The House asked itself, in respect of each allegation, whether, applying this newly established test, the double criminality rule was satisfied. By the time the House of Lords heard argument the Spanish courts had decided that they had (and presumably, although it is not clear from the reports, would at the time of commission of the alleged offences have had) jurisdiction over all the alleged crimes. So there was no problem about the Spanish side of the double criminality requirement.
On 10 December 1984 the United Nations Convention against Torture and other Cruel, Inhuman or Degrading Treatment or Punishment ('the Torture Convention') was enacted. The United Kingdom and Spain were both signatories to it. Section 134 of the Criminal Justice Act 1988, which came into force on 29 September 1988, created a new crime in English law, the crime of torture. It gave the English courts jurisdiction to try allegations of torture, wherever the alleged crimes were committed. Before then the acts which constitute torture would have been criminal in England (e.g. causing actual or grievous bodily harm), but acts of torture committed abroad could not be tried in England.
Accordingly the double criminality rule was not satisfied in respect of those allegations relating to torture which related to times before 29 September 1988. …