The small red/blue temperature plastic disc beneath the tap lever can be prized out to reveal a grub srew. Release this and the lever will lift away and the domed plastic cowel will unscrew. This reveals the main large retaining nut - unscrew and lift out the plastic monoblock. Remember to turn off the water and cover plugholes first! If the action is stiff it is likely one of the rubber seals on the underside is damaged and fouling.
John Sattaur
August 2009