Hi, This should be a really simple question for a plumber so here goes. Underneath the kitchen sink is a fitting for a dishwasher waste pipe. However, there's no dishwasher at the moment. Occasionally, water comes out of there. What's the best way of capping this, please? I tried using one of the other two from the sink outlets. The white nut goes on fine but the blue cap seems to be 2mm or so slightly smaller. Thanks, Ed.