DEEARR2: Why would there be a drain point outside ? i've never seen one. It's a vented system so if you meant a pressure relief valve - there wont be one, and the PRV is not meant to be used for draining the system, unless in an emergency.
Turn off cold water, isolate one radiator (smallest downstairs) undo nuts and lift radiator off - connect hose to radiator valve, open and drain. Use this opportunity to fit a radiator valve with drain point incorporated.
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