Anyone knows how to dissemble a Secta basin mixer tap?

[JimboMarimbo]

It's dripping, not too bad but steady. Needs a replacement cartridge, I reckon.
But how to take this chunk of polished steel apart without marking it?
I'm assuming that the stationary sleeve conceals the cartridge, but all my attempts to unscrew it, or pull/twist it, have not shifted it. Even tried moderate pressure with a plumber's wrench, having wrapped the sleeve with thick cloth.
The activating lever 90deg bit unscrews, and I was expecting to see a grub screw underneath, but nothing.
I'll upload a couple of pics for clarity, and would welcome any help.

Thanks in advance.

Secta basin mixer tap supplied by Victoria Plumb Dec 2015 - no longer stocked!

 

[picasso]

If there is no grub screw then its probably just pressed on, try gently tapping the top bits from underneath, the collars further down normally are screwed to the tap gear so try unscrewing.