On 10/10/2018 22:11, TimW wrote:

I want to keep the trough available for livestock. I have no way of
turning off the water. I don't know if the water is full mains pressure
or if the slow flow is all we will ever get. I don't know if the
existing valve is blocked or not or if it's fixable. I can go to mole
valley or somewhere and buy a new ball valve or similar no problem, 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? What sort of fitting is
needed for black plastic pipe? 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?

That's what a farmer would do. As others have said, you will get wet.
Pipe freezers are for wimps, but that should work too and may be easier
especially if you are not familiar with the fittings. You don't want to
lose one of the bits once the water is flowing!