Inside Kim Leadbeater’s office in Parliament, ‘five’ and ‘zero’ helium balloons are gently deflating. They are a reminder of the weekend the Labour MP for Spen Valley has just had in Harrogate celebrating her birthday, but also carry a more poignant memory. Ten years ago, Kim’s sister Jo Cox was the star guest, arriving last minute from Parliament, throwing off her MP’s suit and embracing the party spirit in a neon tutu. “We had so much fun that weekend,” Kim says. “Jo wasn’t an MP, she was