Having reconfigured the waste plumbing I've ended up with a strap on boss on the soil pipe that is no longer needed. It had a 40mm pipe going into it with a rubber adapter so I just pushed in a 40mm blank, job done. Some time later I found the blank on the floor so I pushed it back in. Its now slowly working it's way out again. Is there a better fix? Can't find a solvent weld bung.