I need to take out one of my radiators in the house as I'm plastering behind it.
Am I right in thinking that the radiators are connected in parallel to the 'supply pipe' and I can just cap the ends off or will it stop the flow of the water?

just turn of the valves to each side of the radiator-uncouple from rad and drain it off.
a one pipe system is very rare and i would duspect you don't have one.hth rich