On Wed, 10 Oct 2018 22:11:14 +0100, TimW wrote:

but lets say I cut the pipe with a hand saw and water gushes out. Will I
be able to fit the new valve with water flowing?

Yes, but you'll get wet, probably very wet.

What sort of fitting is needed for black plastic pipe?

Depends on the OD of the pipe. Ordinary compression will work (on the
right sized pipe) but you really need to put a metal sleeve inside
the pipe as well. There are also various other connectors for plastic
to copper connections.

I had an idea that I could cut the pipe and maybe fit a stop cock with
the stop cock open and the water flowing through, then turn it off, then
deal with the trough valve. would that work?

Yes, but you'll get wet, probably very wet. With the valve open you
have water coming out of it, proably quite a lot of water. I'd have
the valve closed, then when you shove it on the end of the pipe you
have a much lower flow rate,

I'd use a "squeeze off" tool to compress the pipe to stop or very
much reduce the flow before cutting the pipe.

--
Cheers
Dave.