One way to do this if you are using Tableau version 7.0, is to go to Map-->Edit Locations...
Then, double click on the matching location, select the drop down menu and select Enter a latitude and longitude...and input the lat/long accordingly.
If you have the latitude and longitude fields in your data already, then just be sure to put your latitude and longitude fields on the row and column fields instead of the Latitude (generated) and Longitude (generated) fields. You may need to set the geographic role of your fields.
If you have the latitude and longitude in a separate file, then use data blending to blend that table on the postcode field onto your table. Then do the same as above.