Removing old Frisone basin waste

[HairoftheDog1]

Hello
I am about to replace the basin waste in a bathroom. The old fitting is a Frisone pop up but I am unable to see how to remove it. All other basin wastes I have ever seen have a large 6 sided 'nut' that screws onto the fitting from under the basin and then tightens the whole thing in place. There is not one on either of the identical Frisone fittings that I have. There is simply the metal fitting (without any thread until the end 1/2" or so) that feeds through the plug hole which is covered with a rubbery washer tight to the bottom of the basin. Nothing to unscrew that I can see to enable removal.
Does anyone know how to remove the fitting?
Thanks

 

[Newboy]

If there's nothing holding it onto the underside of the basin then my guess would be that it's been bodged and glued in place with silicone or similar.

Edit - it's possible that a basin mate has been used but without the backnut - try unscrewing the big rubber washer