The group fend off competition from Nicki Minaj's 'Starships', which climbs from four to two this week.
Elsewhere in the top ten, Skrillex advances from nine to five with 'Bangarang', Chris Brown rebounds 10 to nine with 'Turn Up The Music', and Taio Cruz lifts 11 to 10 with 'Troublemaker'.
Further down the chart, Carly Rae Jepsen vaults 39 to 15 with 'Call Me Maybe', Labrinth and Tinie Tempah's 'Earthquake' bounds 28 to 21, and Taylor Swift and The Civil Wars' Hunger Games song 'Safe & Sound' debuts at 38.
Adele's 'Someone Like You' re-enters at 44, and Ed Sheeran's 'Lego House' starts at number 49.
The Top 10 singles in full (click where possible for our reviews):
1. (1) fun. ft. Janelle Monae: 'We Are Young'
2. (4) Nicki Minaj: 'Starships'
3. (3) Flo Rida ft. Sia: 'Wild Ones'
4. (2) Gym Class Heroes ft. Neon Hitch: 'Ass Back Home'
5. (9) Skirllex: 'Bangarang'
6. (5) 360 ft. Gossling: 'Boys Like You'
7. (6) Qwote ft. Pitbull: 'Throw Your Hands Up'
8. (7) Jason Derulo: 'Fight For You'
9. (10) Chris Brown: 'Turn Up The Music'
10. (11) Taio Cruz: 'Toublemaker'
Watch fun.'s 'We Are Young' music video below: