Normally a lug nut at the side. But if you find a similar mixer on a plumb8mg website you can follow the installation instructions in reverse.

Is the black housing the nut?

There doesnt look like there is a nut anywhere. ive tried to twist the black plastic and it doesnt work.

The mixer is joined to the flexible hoses. You disconnect the flexible hoses from the main connection under the sink, in the cupboard. You then pull the entire mixer and attached flexible hoses up through the sink hole.

Extract from bathroom direct.

Turn the water supply off completely at the stops and check that it does stop the supply
Undo the hoses from the stops with your spanner. They'll usually unscrew fairly easily by hand once you've loosened the grip
Loosen the nut under the bench that's holding the old mixer in place using your ratchet hex set or spanner
Once you've done this the old mixer tap is ready to remove.

Simply lift it out of the hole slowly, bringing the hoses with it. You may need to fiddle around with the ends of the hoses to bring them through the hole one by one. They certainly won't come out at the same time because of the fittings at the end of each hose.

Usually on new taps the flexi hoses attach directly to the base of the mixer tap. It appears from your photo that the flexible hoses have been connected to some form of piping, not to the base of the tap. This does not look correct, and you did say it was old, so maybe the previous person who attached the new flexible hoses had to improvise. Therefore no nut to hold the tap in place. I would be consulting a plumber to attach all the new components correctly, and remove the old tap components and extra pipes.

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Thanks dreamer. Exactly thought id find a nut ad pictured but nope. Called in a plumber to fix as worst case will need to grind off the old system. very odd situation in my mind.

Thanks again community