If water is draining slowly or failing to drain from a basin, sink or trough there are some things you can try before calling the plumber. First Check the gully drain associated with the sink (see diagram). If it is overflowing you'll need to call a plumber. However if it isn't, then it's highly likely that the blockage is between the sink and the gully drain, in which case you can try the following to free it up:
Try using a rubber plunger to free the drain blockage. Make sure the cup of the plunger is making a good seal around the plug hole then move the handle vigorously with a straight up and down motion. If this doesn't clear the blockage, go to step 2.
Check the S-bend trap (see diagram) by removing the screw-on cover and check for blockage there. Be sure to place a suitable receptacle under the bend to catch any water or debris that might pour out. If the problem is not in the S-bend trap, go to step 3.
Outside the wall there will very likely be an external down pipe with an inspection opening (see diagram). Remove this and run water from the sink. If water flows out of the opening it's likely that the blockage is between the it and the gully trap. You can try inserting a garden hose first from the inspection opening end and then from the gully trap end (after removing the gully trap grill) and turning on the water at highest pressure to free the blockage.