Some of the valves turn 90 deg to off, you can then turn 90 deg to on etc, but just keep turning in the same direction, on/off/on etc - ballovalves ?
Have to get the off position in just the right position.
There is a handy kit - bung kit - that is like two rubber cones, with a 15mm or 22mm hole in the fat end, cone goes into the outlet of the heating header tank, hole fits over the vent pipe that hovers over the tank, if you are lucky, with these in position and a small water loss you can change a joint on the heating system (only one joint at a time).
Edit:
http://www.heatingcontrolsonline.co....kit-p-211.html something like this..