I have a double sink, and the small sink is about 29cm x 15cm, and 12cm deep. When filled, the water slowly drains away. I tried a simple rubber plug, and the water does not drain away. This sink is not used much, so I don't think that it is down to wear. The plug is the original plug that came with the sink. It is an ivory colour, which matches the sinks. I could purchase an exact replacement, but I am conscious that I may have the same problem. Below is a photo of the plug.