Adam Funk wrote:

What's the correct procedure for turning off the flow of hot water
from the boiler in order to cut the pipe and put a service valve in?
Should there be a valve somewhere downstream of the boiler, or do I
turn off the main stopcock coming in?

There's no "correct" procedure. Just prevent the water flowing!

There's usually a valve on the cold water inlet of most combis.
Otherwise just turn off the cold on the stopcock.

No need to turn the heating off either.


--
Scott

Where are we going and why am I in this handbasket?