Who Is Responsible?
Please note that the following information is just my interpretation of what I have researched and been told. I recommend that you consult with your Local Water Authority.
Since October 2011 the laws regarding drains and sewers that eventually discharge to a Public Sewer or Public sewage treatment plant have changed, as I understand it, to the following:
Drains serving more than one property (sewers) that connect to the mains sewer system have become the responsibility of the Water Authority from the point at which they join together - even though they may be situated under private land.
Drains serving just a single property are still the responsibility of the property owner whilst within the boundary of the property. Once they pass beyond the boundary of that property or join with the drains from another property they become the responsibility of the Water Authority.
So, basically, the Householder is only responsible for those drains serving just their property and whilst on that property.
Drains and sewers connected to private sewage treatment plants/septic tanks are not included in this for the time being.
The situation with storm and surface water drains is not so simple.