Ideal Standard mixer tap very stiff?

The mixer tap has become very stiff in use especially when running hot water. I thought I might grease it but I can't even get to the cartridge to look at it. I removed the flat top that just levers off in two parts - the chrome disk with a black plastic disc under that. I'm left with the chrome tap with two slots either side and I cannot see how to remove that to get at the cartridge.