James Blunt has performed a duet of 'Grease' classic 'Summer Lovin'' with ex-lover Paris Hilton.The British singer was joined on stage by the socialite at bikini designer Diana Jenkins' birthday party in Malibu on Saturday night.The unlikely collaborators' impromptu rendition of the popular hit is said to have gone down well with revellers, who danced and sang along.After finishing the song, Blunt was seen leaving the lavish party with German model Tatiana Patitz.The pair had been flirting earlier in the evening and seemed to be smitten with one another as they left the bash.Blunt and Paris have been spotted kissing and dancing in nightclubs on several occasions this year, although neither star has ever confirmed they dated.The 'You're Beautiful' singer has previously dated Czech model Petra Nemcova and has also been linked to Mischa Barton and Lindsay Lohan.Paris has most recently been seen getting cosy with Pamela Anderson's ex-husband Kid Rock.