Nylon tap washers are a popular option for sealing tap valves closed.
I bought some in Tescos on the strength of the label, and found that they were actually the wrong size, 5-10mm too big.
Unfortunately, they look like they have been stamped out by some kind of Play-doh tool.
Again two days later tsp was dripping again.
The washer moves up and down when the tap is turned in either direction, turning the taps on when it moves up, and off when it moves down.
Beveled units are flat on one side and have an oval, sloping angle on the other side.