My bathtub is leaking water into the main bedroom from underneath?

I removed the drain plug thing ( metal thing in bath ) and saw a gap 100mm before the next pipe. I placed flexible pvc piping in the gap but still have the problem, he draining is alwfully slow now. Builder wont take action due to it apparently being finally inspected and was classed as " All Good " house is 10 years old and i have mould growing in my walk-in closet due to it.