Childhood pen pals in London, actress Carey Mulligan and singer Marcus Mumford lost touch long ago, but a recent reunion renewed more than friendship.

After five months together, the Mumford & Sons singer, 24, popped the question to the Never Let Me Go star, 26. "They're a great couple," a pal tells the new issue of Us Weekly (out now). "Very sweet."

Things first got serious when Mulligan and pal Jake Gyllenhaal caught a secret Mumford & Sons show in Nashville, Tenn. February 7, at the home of band members from The Apache Relay.

"At the end of the night there were just the musicians, Jake, Carey, and the Mumford boys," an insider told Us at the time. "We had some pizza, and just played music. Jake picked up a guitar and Carey joined in on 'Amazing Grace.' Marcus and Carey seemed really friendly."

A few days later, Mulligan and Mumford caught a secret Arcade Fire show in NYC before hitting up a private members only club, a second insider told Us.

Oscar-nominated Mulligan split with longtime beau Shia LaBeouf last October; Mumford broke up with singer Laura Marling in late 2010.