I think you might be thinking of Habeas Corpus, which came about in part through common law and later in the form of the Habeas Corpus Act, some 400 years later

I think you might be thinking of Habeas Corpus, which came about in part through common law and later in the form of the Habeas Corpus Act, some 400 years later than the Magna Carta Act. The most important part of the Magna Carta Act is the guarantee that you will be tried by your peers. In civil cases at the moment in the UK there is a strong resistance to have secret evidence even though the current Justice Minister suggests that some evidence should be withheld from the accused! So common law cases, like Habeas Corpus actions, are still tried in front of your peers with open evidence. This may change if Ken Clarke, and according to some accounts lobbying from the US CIA, have their way.