i dont know a lot about rads, and cant make sense of how the valves
stop the water circulating the rad. how does the flow of water get to
the 'out' pipe? the vale seems to be outside the rad so i cant see how
it stops it.

anyway main question is, is it easy to remove a rad off the wall so
that i can get behind it, as it will need replastering? do i need to
stop the water supply completely and drain the system or anything?

i have a main water stop tap and this supplies a combi condensing
boiler which heats the water and central heating.