The Spice Girls are set to reunite at Geri Horner’s birthday party. The ‘Bag It Up’ hitmaker will turn 50 in September and not only has she invited Emma Bunton, Mel B, Mel C, and Victoria Beckham to the lavish bash she is planning, the quintet are expected to perform together as a five-piece for the first time since the closing ceremony of the London Olympics in 2012. A source told The Sun newspaper: “It’s going to be an amazing night. “It’s a seated, black tie do and the theme is Timeless Elegance. “Loads of Geri and her husband Christian’s celeb pals will be there and, excitingly, Geri is in…