Opening up a Vernon Tutbury tap?

The washbasin in my bathroom has some attractive gold and chromium taps. I want to remove the cap cover to get at the working parts. Can anyone help? There is a gold plated and slightly domed cover that looks as if it ought to come off, but I am frightened to damage the plating. Similarly there is a gold plated collar that unscrews but doesn't seem to loosen anything. Any ideas out there?