About a week ago I noted a faint hiss coming from inside of my under-stairs cupboard. Over a few days the volume of the hissing increased.
I removed almost everything from under the stairs and piled it high in the hallway. Turning off the stop-c*ck inside and I could still hear the hissing. It stopped when the outside stop-c*ck was turned off. Cut and removed one floorboard under the stairs - underfloor space very wet with water running down inside of the foam pipe insulation.
Problem eventually found BUT under everything I had piled high in the hallway and required lifting much of the fitted carpet and underlay. The copper pipe had a pinhole leak which became much worse when I ran my hand along that section of pipe.
I replaced a 2 metre length of 22mm pipe and two 90 degree bends.
On close examination the copper pipe appears to have gone rusty! There appeared to be a a slither of different metal about 0.5mm by 2cm within the wall of the pipe and a thin layer of copper in the area which has delaminated . This pipe was originally fitted by me around the mid 1980s possibly from a batch of poor quality pipe that was imported at that time.