"To drain down I assume you turn off the hot water / heating system, turn off the mains under the sink and run the hot water until it stops."
If you do that you'll empty the CWST in the roof. You may find that you have a valve fitted to the outlet from the tank and / or on the feed into the cylinder. Shutting either or both of these will take the pressure off the cylinder and flow should stop. There should be no need to shut the main stop c o c k under the sink. Once you have done that open the tap you want to work on AND the HW tap in the kitchen. Then therev will be no water in that pipework at all.
There is a risk, as HE says, that if these are gate valves then if you use too much ignorance then they could sheer of in which case you are looking at a drain down. Touch wood its never happened to me though. If the wise man that fitted them used lever valves or stop c o c k s then there'll be no problem.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!