Check with any of the DIY outlets or local plumbers shop see if the do 'head replacements' for your mixer and replace both. Depending on the type: Remove centre cap and handle screw remove handle Udo tap head.
Capstan types have a side screw then undo chrome cover, use a cloth and wrench cloth protects the chrome. remove tap head.
Take head to any plumbing outlet in most cases better with the small shop they are more knowledgable for a replacement tap head.
BB
November 2009