All five members were originally reported as expected to be reuniting for the milestone occasion - however, it ended up being just four, with Victoria Beckham, Emma Bunton, and Mel C all posing happily with the birthday girl.
It’s also believed that the former girlband members gathered to sing some of their old classics at the star-studded soirée.
Instead, Mel B opted to stay home and celebrate her recent engagement to hairdresser Rory McPhee, while enjoying Bonfire Night with her loved ones according to family tradition.
Mel B’s representative told The Standard: “Melanie attended her family’s annual bonfire night event which was also to celebrate her engagement to Rory - but she had a long chat with GerI before the party.”
Horner spared no expense on the lavish party and, according to The Sun, dropped a whopping £400,000 on the bash.
She officially turned 50 in August and had planned for the party to take place in September, but decided to postpone it following the Queen’s death.
Entertainment was provided by Dame Shirley Bassey, 85, who belted out Happy Birthday.
Former X Factor winner Alexandra Burke is also believed to have performed.
Also in attendance was TV presenter Rylan Clark, who shared a group picture with Horner, her husband Christian, daughter Bluebell, 16, whom she shares with director and screenwriter Sacha Gervasi, plus her bandmates.
The star-studded guestlist included Myleene Klass, Poppy Delevingne, Chris Evans, Holly Willoughby, and Zack Ashley.
According to the publication, Christian made a heartfelt speech at the party and was also snapped lovingly slow dancing with his wife on the dancefloor.