In general the only drains that you are responsible for are the lengths of pipe within the boundary of your property that serve only your property. With the exception of private pumping stations, septic tanks and the drains to them, which remain the responsibility of the users of them.
If you have a blocked or damaged drain within the boundary of your property and the drain only serves your property, you are responsible for unblocking or repairing it.
If the blockage is outside the boundary of your property or is in a pipe serving more than one property the water authority is responsible and you should contact them directly. Further information about who is responsible for drains and who to contact can be found on the Thames Water website.
The storm drains and road drainage are the responsibility of the county council and any problems should be reported to them
Drains can become blocked if the wrong materials are put down them. For further information see Bin it - don't block it.