Timothy Murphy wrote:
Is there any method (short of divining!)
for determining the course of an underground water supply?
[The outdoor stopcock appears to have been asphalted onver.]

If it's metal, you can use a cable detector; you could probably hire
one. Some cable detectors come with a separate transmitter which is
attached to the pipe with a crocodile clip and sends an electronic
signal down the metal pipe. The detector unit is switched to the
relevant tracing mode and can detect this signal.

If it's plastic, then I haven't got a clue.