Sometimes no matter how hard you try you never seem able to plumb in a waste without it leaking....
This is a sprung basin waste and it works by pressing the centre of the waste fitting in the basin, when it pops up the water is released from the basin.
The kit comprises of the sprung basin waste, a sealing washer and a plastic nut.
You can see here that there are holes cut into the side of the waste, these are for the overflow water to escape into the waste pipe as most porcelain basins have a built in overflow. It is important that these holes are not blocked.
Sometimes no matter how hard you try you never seem able to plumb in a waste without it leaking. This is a common problem caused by irregularities in the basin hole preventing the waste washers from forming a proper seal
The solution I have is to leave out the thin foam washer that's meant to go under the flange inside the basin and run a bead of white or clear silicone along the underside of the flange instead - make sure there's enough so that it gets squeezed out all the way around with no air pockets.
Underneath, run a bead of silicone on both sides of the thicker rubber washer. Once tightened up, smooth the silicone around the nut and washer. Inside the basin, remove as much as possible, then use a rag wrapped around a finger to wipe the rest away leaving only a near-invisible ring around the flange.
Another option is to buy a basin bowl sealer kit from B&Q for around a pound. The manufacturer is Robimatic plc. Using one of these is a great way of easily sealing the basin waste without the need for tape or messy sealant. The kit comprises of a white foam washer, the basin mate tapered rubber washer and a poly washer.