At the back of the handle there is a 2.5mm allen key grub screw - undo that and lift off the handle from the cartridge spindle.
You will see the cartridge inside the tap barrel secured by the hexagonal retaining nut. Now lever off the dome shaped chrome surround that clips to the body of the tap by using a small screwdriver or similar to gain access to the hexagonal nut.
Talkingbollox, January 2009