If you have a water meter (did you say?) then have a look at it. You may find there is a glass window (rub it clean) with a bubble of air in it. all the time the water is flowing, however slight, the bubble will turn. So if you turn off your indoor stopcock, and the bubble is still moving, you have a leak. Also look at your neighbours stopcock. you will need a torch. The little hatch probably opens wih the handle of a teaspoon, or an old screwdriver. If the pit is full of rainwater or mud, spoon or ladle it out.
You can also detect a leak by listening. If you know a young person with sharp ears, get them to stand in the kitchen late at night, when it's quiet, while you turn the pavement stopcock (there is one on the water meter) on and off. If there is a leak they willl detect the almost inaudible background noise stopping and starting when you turn the cock off and on.