He doesn't. A house owner is responsible for any pipes, supplies and drains on their property even if they do not originate or terminate on their property. The responsibility of the water board stops at border of your property.
I don't think this is right actually.
I believe the property owner is responsible for the length of his pipe even if it crosses another persons land.
This is different of course if its shared, which again isn't your land your responsibility, it depends on the layout, if its on your land but is shared, its shared.
This helps, its colour coded to show responsibilites. They don't have a specific one to show a pipe simply crossing, but the table says its property owner, note this doesn't mean land owner it means the supplied property owner
I think this has changed a few times
https://www.ofwat.gov.uk/households/supply-and-standards/supply-pipes/
EDIT: actually it does cater for this.
Look at the bottom of the water supply picture, where the right most property is soley responsible for the pipe on the adaject property once it becomes a sole supply for the rightmost property.