How can I remove tap heads from a Carron Angel mixer?

I have a Carron Angel 1/4 turn two lever mixer tap. I cannot see how to remove the tap heads to get at the disc assembly. What I thought was a grub screw turned out to be a spring loaded button. I have pushed this button all the way in and then tried to ease off the tap head. It won't budge! Do I use more brute force or is there another way of removing the tap head?