Leak under the sink

[cattypussclaw]

So, I have a leak under the sink (see photo). Currently trying to find someone to come out and look at it.

It is a join on the black pipe that is leaking. I can't reach the stopcock outside (too deep) and the kitchen one has no effect on the leak. How can this be? Can anyone (Piglet John) tell me what the black pipe is that disappears into the concrete floor?

Thank you.

Just bumping this as the link wasn't working the other day

Does the pipe disappear into the ground? If so it is your incoming water pipe so you will have to close it off at the external stop cock.
If the tap is too deep then you will need a long handled key.
I take it you don't have a water meter or modern stop cock otherwise it would be much closer to the surface and within easy reach.