Thanks Bernard, i was wondering why that wasn't the case. If they had been then i'm sure that this port wouldn't have leaked even with the top pin released.
I was wakened in the early hours by the large upstairs hallway radiator making sloshing/gurgling sound. Being half asleep i just turned it off thinking air etc, but it kept on gurgling so i headed downstairs to the understairs cupboard where the heating manifold and pump are located and...