I understand it he was brought back as they wanted to hand him over to Sweden and he had to be sent back because of him being a combatant and the Geneva Convention said so.

Quite correct.

Captain Alfredo Astiz was brought to the UK at the request of France and Sweden in connection with:

“the disappearance of Dagmar Ingrid Hagelin, a 17-yr old Argentine-born girl holding Swedish citizenship, and two French nuns, Alice Domon and Léonie Duquet, the three of them in 1977” (Wikipedia)

Due to his status as a POW the UK was unable to extradite him to France or Sweden, although he was questioned by British police but remained silent. France convicted him in his absence in 1990. He was repatriated to Argentina before the end of the Falklands War.

http://en.wikipedia.org/wiki/Alfredo_Astiz