If "pin" is the Thermostatic Valve pin it's the little yellow thing just above the collar in this picture...
http://www.diyfixit.co.uk/diy/building/remove_radiator/thermostatic_valve.jpg
You undo the collar - lift off the top and gently pull up with pliers.
If you mean the bleed screw it's here...
http://www.dover.gov.uk/privatesectorhousing/graphics/valve.jpg
Peccavi, February 2009