Sarah teamed her newly-brunette locks with a pretty black dress with a corsage neckline and a retro-vintage hairdo.
She was joined by her bandmates Nicola Roberts and Kimberley Walsh at the upmarket Brompton Club, to celebrate her engagement to Tom Crane.
Kimberley joined Sarah and Tom were seen checking into the nearby Hotel Kensington, while crates of Moet & Chandon arrived at the party venue in the early evening.
Other stars to attend the party included Rosie Huntington-Whiteley and Jason Statham, and Jo Wood.
What do you think of Sarah's engagement party look? Leave a comment below...