Spud1962:
If this is your heating system water you're planning to drain off, then there SHOULD be a drain valve on your boiler. Snoop around your boiler and you should find it. If you can't find a drain valve, but do find a place where you can screw a pipe into your boiler, I would plumb a ball valve in series with a "sillcock" (pronounced "a valve with a male hose thread on the outlet") so you can attach a hose to drain your boiler into a floor drain. The upstream ball valve will allow you to replace a leaking washer in the sillcock during the winter when you don't want to drain your heating system to do that.
Also, use the money you were thinking of spending on putting that hole in your wall (and caulking around it) to install a short 1/8 nipple, 1/8 inch ball valve and 1/8 inch air vent on the two highest elevation radiators in your house. (Make sure you get ball valves and not just petcocks. Petcocks are inexpensive, but they'll seize up before too long, and you'll be replacing them with a proper ball valve.)
With those ball valves in place, when you need to drain your heating system to do repairs you can store the water in some empty 5 gallon pails. Then, after the repair is done, simply carry those 5 gallon pails up to the highest elevation radiator in your house and siphon that water back into your heating system. You just need to replace the air vent screwed into the top of that 1/8 inch ball valve with a 1/8 inch hose barb fitting onto which you can push the siphon hose.
Use the other 1/8 inch ball valve to allow the air in the system to bleed off as the syphon refills the system.
That way, you're not adding dissolved oxygen and hardness to your heating system when you refill it. The dissolved oxygen and hardness will form rust and scale, respectively, inside your boiler.
And, this way you refill your system to the correct pressure even if you don't have a pressure gauge on your boiler (or if you don't know how to use it). Also, siphoning the old water back in also allows you an ideal opportunity to check the concentration of corrosion inhibitor in your heating water and to add more if necessary.
Any place listed under "Hose Fittings" in your telephone directory will either sell 1/8 inch ball valves or know who does.
Here in North America, 1/8 inch is the standard size for radiator air vents. I don't know if they use a different (or metric) size in the UK.
