turn the water off at the stop cock ,undo the top this could be fixed through the top under the h/c logo then undo the cover this will let you see the large nut this holds the top of the tap with the washon it in place 2 sets of grips are required so you can hold the base of the tap while undoing the top once it is off you can change the washer or get a new insert.
robert
May 2010