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To confirm is it the washer at the base which is slipping so the entire tap is rotating a the base, which is the issue?
Please respond here to the question, I have asked customer services to close the duplicate.
As the entire tap is rotating, I would think you do need something thats going to stop it slipping (such as a star washer) on the underside of the sink, this certainly wouldnt hurt, Maybe also a lock nut if the nut is coming lose as well.
These are not normally supplied with taps as standards, and if you not seen other reports of this model of tap having the issue it has to just be down to the combination of tap fitting and basin.
So would go down to your local DIY depot and buy a star washing of that required size, and another nut and see if the two help hold the tap in place.
It wouldnt hurt, but cant see it would give it the level of friction required in my view.