Their debut Album sold a million copies and won the UK and Ireland's Mercury prize in 2012. Now on their third recording, "Relaxer", they’re one of Britain’s most successful bands – although if you bumped into them in the street, you probably wouldn’t know it. Eve Jackson caught up with Alt J’s Joe Newman and Gus Unger-Hamilton to talk about Sex Clubs, selling out the world’s biggest stadiums and going on celebrity Mastermind.