turn the valve head to the lowest setting and directly beneath this is a grooved ring, turn this anti clockwise use water pliers if it is stiff, dont worry no water will leak.
the head will come off and you will see the pin using the pliers grip the pin, push inwards, if it will not go in the the valve is stuck. so pull the pin outwards then push back in. note then pin might come out and water will trickle out, do not panick push the pin back into the hole, job done. put the cover back on.
malcolm link, April 2008