A drain takes wastewater and sewage from your sink, bath and toilet etc, direct to the sewer.
It is the responsibility of the property owner to maintain and keep the drain running freely and for the clearing of blocked drains.
You may find that your toilet is blocked, grids that take water from sinks may overflow or there may be smells.
Check if your neighbours are experiencing similar problems. If none of your neighbours are affected, it is likely that your drain is blocked and therefore the responsibility lies with you.
If you are unable to unblock the blockage yourself contact a specialist drainage contractor. If you are a tenant you should contact the landlord or Housing Association.
If your neighbours are also affected the problem is likely to be with the sewer and the responsibility lies with Thames Water.
Look at Thames Water for further help and information.