The 25-year-old pop star was one of two special guests at the charity bash, which was hosted by the BBC Radio 1 DJ, at north London's Camden Roundhouse.
Mouths dropped as the 'Too Good At Goodbyes' hitmaker casually took to the stage to sing the dance track, which he recorded with the electronic duo - comprised of brothers Howard and Guy Lawrence - in 2013, a year before Sam became an international star with his debut record 'In The Lonely Hour'.
Prior to Sam's appearance, the likes of Stefflon Don, Mabel, Raye and Wretch 32 all performed and helped raise £30,000 for the Roundhouse charity - which supports young budding artists in achieving their dreams.
Another special guest was Years And Years frontman Olly Alexander, who did renditions of his band's hit songs 'King' and 'Desire' with two backing singers.
However, it wasn't as much of a surprise as Sam turning up, as the singer took to Twitter to tease his appearance at the sold out event hours before.
The evening also saw some of the best rising talent in the grime world, including 'Addison Lee' hitmaker Not3s - who also performed 'My Lover' and 'Fine Line' with Mabel - and 'Know My Ting' star Ghetts entertain.
The night was not without drama, as within minutes of Ghetts coming on stage, the building was evacuated with gig-goers not being told what was happening.
However, Annie took to twitter to announce the party was back on and explained it was simply a smoke alarm that had been set off - and the line-up resumed.
Joking that Ghetts' was ''too good'' and shut the place down, Anne wrote: ''The party is back on after a slight blip - basically @THEREALGHETTS was too good and we had to evacuate the building @Amp (sic)''
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