depending on the room under the floor it is possible to cut the pipe live and fit a new valve, it gives you a bit of a buzz but its surprising how little water you spill, especially if you can beothered rigging up a hose or something onto the outlet side of the valve and running it to a drain or outside, (it helps when you are trying to push the new valve onto the pipe if the water is running through it), if you just have the valve and it is shut the water pressure can build up a bit and cause some to spill out, just make sure the valve is fully on the pipe if you do it live as you dont want it blowing off later!!!!!!!!!!! when on and shut you can then rig up some pipe alterations and fit a second valve above the ground for future access, if you want to freeze it you need about 6-8" to fit the insulation sleeve and cabl ties to insert the freeze nozzle,