Prepare a container, ideally a square plastic container such as an old ice cream tub. Partially loosen the instantor nut beside the radiator using your spanner, while taking care not to put undue strain on the copper pipe while doing this. It is a good idea to hold the copper pipe when operating the spanner to reduce any stresses that may cause a fracture in the pipe. Bring the plastic tub under the joint and fully loosen the nut. Pull the joint apart slightly to allow the water in the radiator to drain into the tub.