Generally speaking, you're usually responsible for drains inside the boundaries of your property, while the sewerage company is responsible for lateral drains, which are outside of property boundaries, and sewers. Although most sewers are now publicly owned, there are still some private or unadopted sewers. If your property is served by one of these, you may be responsible for maintaining it.
What's the difference between a drain and a sewer?
A drain is a pipe that drains rainwater from a building and other buildings which belong with it, for example a garage.
A sewer collects water and waste from the drains of a number of buildings. Most sewers are publicly owned and are maintained by your water company. However, there are still some privately owned sewers. Some people aren't connected to a sewer but to a cesspool, septic tank or treatment plant. If you aren't connected to a sewer, you won't have to pay sewerage charges to a sewerage company.
Repairs to drains
You are responsible for maintaining or repairing any drains inside the boundaries of your property.
You will have to pay to get this work done, but you are free to choose whichever company you want to do the work. Alternatively, you may take out insurance to pay for work on private drains.
Repairs to sewers
Sewers and lateral drains connected to the public network used to be the responsibility of the property owner. However since 2011, most are now maintained by local water companies. If you have any problems with your sewer or lateral drain, for example if it's blocked, contact your local water company.
More information from Southern Water – https://www.southernwater.co.uk/sewer-ownership-changes