How the heck do I get this apart? Mixer tap

The mixer tap in our kitchen is leaking around the base. I've taken the top cover off, and undone the nut underneath the sink, but now I'm stuck! The thing that looks like a grub screw on the shank is not a grub screw - it has no slot for a screw driver. The thing on top has some flat sides so I thought it might be a nut, but when I turn it, the whole thing turns (ie twists the copper pipes underneath). So what's the story here?

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