Hi Nigel
we had blocked drains, water coming back up from the sewer in our kitchen sink, when they come to fix they flooded mine and both my neighbours with rotten water! anyway the plate in the bathroom if you unscrew it there should be a rubber seal, if not its missing, you can get one from brico, put it on and tighten the cap up.
then i would suggest you get some silicone and build a rubber seal up on the metal plate let it set then screw it down, ti would prevent any leakages if step one failed.
hope this helps, if your not sure ill send my husband up to do it for you!
regards
nikki