I need to fix a hot tap in a sink in a relative's house (either clean
out & replace the washer, or perhaps replace the tap) --- that's
fine. But there's no service valve for this tap, and she has a
combination boiler.

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?

Thanks.