legislation.gov.uk has the 1297 enactment (25 Edw 1), confirming and reciting an issuance of Henry III, here. Open "Opening Options" and click "Open whole Act" to see the whole thing as amended.

The text at the top, "Changes to legislation: There are currently no known outstanding effects for the Magna Carta (1297).", going by the "?" text, appears to mean that there are no known outstanding edits, where they haven't gotten around to updating the text of the act below.

This shows the text as currently amended (and translated from Latin into modern English). For almost all of it, they just list a clause number and the act(s) that repealed it. Unfortunately, the "Original (As enacted)" button is disabled, so I don't know how you can see the original, except a few images at the bottom.

They list three clauses still in effect (other than the intro):

I Confirmation of Liberties. FIRST, We have granted to God, and by this our present Charter have confirmed, for Us and our Heirs for ever, that the Church of England shall be free, and shall have all her whole Rights and Liberties inviolable. We have granted also, and given to all the Freemen of our Realm, for Us and our Heirs for ever, these Liberties under-written, to have and to hold to them and their Heirs, of Us and our Heirs for ever.

IX Liberties of London, &c. THE City of London shall have all the old Liberties and Customs [which it hath been used to have]. Moreover We will and grant, that all other Cities, Boroughs, Towns, and the Barons of the Five Ports, and all other Ports, shall have all their Liberties and free Customs.

XXIX Imprisonment, &c. contrary to Law. Administration of Justice. NO Freeman shall be taken or imprisoned, or be disseised of his Freehold, or Liberties, or free Customs, or be outlawed, or exiled, or any other wise destroyed; nor will We not pass upon him, nor [X5condemn him|deal with him,] but by lawful judgment of his Peers, or by the Law of the Land. We will sell to no man, we will not deny or defer to any man either Justice or Right.

And the final saving, that starts "General Saving. Observance of these Liberties. Subsidy, in respect of this Charter and Charter of the Forest. Reserving to all Archbishops, Bishops, Abbots, Priors, Templars, Hospitallers, Earls, Barons, and all Persons, as well Spiritual as Temporal, all their [X6free Liberties] and free Customs, which they have had in time passed. ..."

(EDIT: And, of course, they're being liberal in their interpretation of "still in effect". For Some Value of "Rights and Liberties" of the Church of England, and some liberties and customs of the City of London are gone.)

The oldest law still in force is the 1267 Distress Act. There was talk about repealing it, but I haven't been able to find any evidence that it was.

There were many versions of the Magna Carta the first famously drawn n 1215, however it was not copied onto statute rolls until 1297, so technically that means the 1267 Distress Act predates it.

Source: http://www.bbc.co.uk/news/uk-politics-30334812

www.justis.com (could not get a durable url for the act unfortunately)