My son has been told by the water company that there is a leak in the
pipe supplying his house.

They dug up the road outside and somehow determined that the leak is
in the section of pipe that is his responsibility.

The house is a victorian terrace with about a metre from the pavement
to the front door.

The mains feed is from a blue plastic pipe through to the back of the
house, so it has been replaced fairly recently.

I would have thought that if the leak is serious enough for them to
dig up the road, then it would be obvious with water pouring out in
the house or front path. He says there is nothing obviously wrong in
the house or on the front path.

Where does the householder's responsibility start, is it from the
stopcock in the pavement?

Any thoughts on what the problem is likely to be and how to solve it
would be appreciated.