Vanity unit Pop-Up Waste outlet

We got a new bathroom vanity unit installed about 6 months ago with one of those you beaut pop-up waste outlet holes. I noticed that water was not getting away as quick as it used to so I decided to give it a flush out with the stick plunger thingy. A bit of hair and gunk came up and now the water gets away better. Problem now is , the outlet's chrome cap decided not to stay down for when you want to keep water in the basin. I unscrewed the cap off and after quite a few "clicks" and some...

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