The pipe comes from the drive, under the house, up, into the study (used to be kitchen) where it has the internal stopcock.
So, leak is after this point.
After stopcock it runs along the wall (I can see it as it's been boxed in) about a foot, through the wall, into the bathroom. Again, I can see it all the way as it's boxed in with inspection hatched.
After the bathroom it heads into the ceiling where it splits, half for the boiler and half for the kitchen sink etc.
Boiler: I can mostly follow it to the boiler. Cannot see any leaks nor evidence of leaks (we're still in the ceiling)
Sink: I cannot really trace this but it's still in the ceiling. I know more or less where. It then comes down the inside of the kitchen wall (boxed in, cannot see but no evidence), under the counter (where I can now see it clearly) and up to the sink / dishwasher. Zero leaks or evidence there.
There is also a stopcock on both hot and cold before it Ts off for the outside tap, the dishwasher and the sink. I close that.
So... While I cannot see the pipework in it's entirety it is all in the ceiling or places I can see it. Not once does it go under the floor....
I have isolated the boiler. The cold feed into the boiler (has a T for the fill loop) has a screw-in-the-pipe-stopcock (not sure what you call them?). I close that and that is all I can do. Still, it persists.
What I am going to try tomorrow -
Freeze the pipe where it rises into the ceiling. If it stops there then I have removed 50% of the potential problems.
Freeze the pipe where it exists the ceiling and goes into the box again. That removes 50% of the remaining pipework.
The house is a semi with only one downstairs bathroom. The kitchen is behind the bathroom. There really is not that much to it....