You really need to remove the complete tap insert so it can be dismantled and cleaned - if you turn the water off, and remove the tap top you'll find 2 hexagon nuts beneath. The smaller one is the 'gland' - if you slacken this you may find that things improve but more than likely you'll need to undo the larger hexagon and remove the tap insert. At least you'll need to clean the stem and rewasher it - you may find that a new insert is the way to go here but take the old one with you as there are different sizes. B&Q do a reasonable range.
John
February 2008