The Girls Aloud star – who became close friends with the music mogul after working on 'The X Factor' judging panel together – splashed out on an unusual gift to celebrate Simon turning 51.
The three performers – who were armed with a violin, drum and an accordion – wore grey T-shirts and jeans to copy Simon's sense of style.
However, the tops were decked out with a cheeky message from his pal, reading: "Happy Birthday, Love Cheryl."
As the trio sang the classic 'Happy Birthday' song, even nearby photographers joined in the performance.
Simon was so thrilled by the gesture, he invited the group, actor Phil Holden and friends Ray Griffiths and Darren Horan to spend the day with him.
Phil told The Sun newspaper: "Simon saw us and asked, 'Are you for me?' before bursting out laughing. He thought it was the best present he'd ever had.
"Cheryl said she thought it would funny for us to annoy him.
"But it back-fired a bit because after we finished he said, 'Right then, lads, you're spending the day with me'."
Simon also made a point of celebrating his special day – by splashing out £144,000 on a brand new black Ferrari California sports car.
He smiled: "I got myself a birthday present!"
Simon – who has a large collection of the high-speed vehicles – decided to treat himself to the latest convertible model, which features a retractable hard-top roof, and other customised extras.
