Skwawkbox links to this Guardian article, which itself is sourced from a book from the time. It doesn't seem that far-fetched that a renowned negotiator would be involved with a negotiation.

I just assume every skwawkbox link is misinformation.

If not in an absolute sense then always by ommission.

Google gave me an article from The Guardian from 2001.

Secret UK deal freed Pinochet

A new book alleges the former dictator's release from Britain was brokered between Chile and Downing St.

Augusto Pinochet, the former Chilean dictator, was allowed to escape extradition to Spain on 2 March last year because of plans worked out over many months by Tony Blair and Foreign Secretary Robin Cook in collaboration with Eduardo Frei, then President of Chile, according to leading Chilean sources. José María Aznar, the conservative Prime Minister of Spain and his Foreign Minister Abel Matutes, were involved in the planning.

The Blair-Frei plan was to prevent Pinochet's extradition while observing the law. Instead, the Prime Minister and Foreign Secretary relied on Britain's wide discretion on extradition matters.

The plan was conceived in 1999 after it became clear that the Pinochet affair was dragging on far longer than governments expected and came to fruition when British doctors examined the General at Northwick Park Hospital in Harrow, north London, on 5 January last year. Their report allowed Straw to exercise his discretion to release Pinochet on humanitarian grounds even though the former dictator had never said he was too ill to stand trial.

Frei argued to Blair that neither government would benefit if Pinochet were to die in England and that he could be tried in Chilean courts. According to the book, Blair emphasised to Frei that the case was before the courts and the Government could not interfere, adding that any British leader would court grave problems at home if he were seen to interfere with the course of justice. If there were any powers which Government could exercise they would be exercised by a Home Secretary not a Prime Minister, he said. Blair undertook to do what he could within the law provided the exchanges between the two leaders were kept secret. The authors claim that Blair suggested setting up a 'back channel', with two people appointed to liaise between the leaders' private offices.

The contact man between Frei and Blair was Cristian Tolosa, Frei's press chief, who made six visits to London in the second half of 1999, liaising with Blair's aide Jonathan Powell at Number 10. Yesterday, Downing Street said that it did not comment on contacts between officials.