Pinochet was travelling (in order to have a back operation) on a diplomatic passport. Last week he had been denied permission to enter France on the diplomatic passport. Absent special treaty relations between Chile and Great Britain, *diplomatic* immune status would be governed by the Vienna Convention on Diplomatic Relations. The general rule is that its provisions only cover persons notified to and accepted by the receiving country as diplomats (members of a diplomatic mission) when they enter the country. Otherwise immune status only follows from the persons function. The passport (or in the US even the type of visa granted by the receiving country, a point decided against my side in a case in which I appeared in the US Court of Appeals for the 2nd Circuit in 1984 - United States v Kostadinov) itself leads to no immunity.
There is a second type of relevant immunity in international law, for heads of state - and it has been argued that it extends to ex-heads of state, at least when (as here) the country in question asserts the immunity. The status of this doctrine in the US post-Noriega is a joke (I think the official line is that the Panama government installed by the US did not assert Noriega's immunity).
There are some problems with this type of development (i.e. the extension of imperial jurisdictions in regard to less powerful states) and not recognizing the validity of the "act of state" doctrine (that a country 's courts will not sit in judgement on the acts of another countgry done within its own territory). Here the basis for Spanish jurisdiction is crimes committed against the persons of citizens of the country asserting jurisdiction, and this seems to me a reasonable exception. It is important not to confuse the Pinochet case with (for example) the attempt by the Bundesrepublik after the anschluss to prosecute Markus Wolf - the former head of the DDR's external intelligence apparatus. The key distinction is not merely whether the acts were at the time they were committed "legal" in the state where they were done (Nuremberg settled that) but whether the acts in question were, at the time in question, also being engaged in as "legal" by the state asserting jurisdiction.
By this standard, for instance, arresting Pinochet in the US would violate international law so long as Kissinger goes free. And if Pinochet decides to talk to the Judges, Kissinger had better cancel the plans for his next trip to Europe.
John Mage