From the post:
Magna Carta is one of the world’s most influential documents. Created in 1215 by King John and his barons, it has become a potent symbol of liberty and the rule of law.
Eight hundred years later, all four surviving original manuscripts are being brought together for the first time on 3 February 2015. The British Library, Lincoln Cathedral and Salisbury Cathedral have come together to stage a one-off, one-day event sponsored by Linklaters.
This is your chance to be part of history as we give 1,215 people the unique opportunity to see all four Magna Carta documents at the British Library in London.
The unification ballot to win tickets is free to enter. The closing date is 31 October 2014.
According to the FAQ you have to get yourself to London on the specified date and required time.
Good luck!