Go to Movable Type, located in the Resource section listed below. Here you will find a Java program that will do the conversion for you. Simply enter in the desired latitude or longitude, and the program will convert that to a point on the national grid. Reverse the process to convert a point on the ordinance survey to a longitude and latitude point.
Head to FieldenMaps, also located in the Resource section. This website offers several different conversion options. The one you want to use is at the top left. Enter in the coordinates, press "Convert" and the program will convert that into Ordinance Survey coordinates. As of February 2011, the program is free to use.
Download one of the mapping programs located at MDFS, in the Resource section. In particular you want to download the NGRCalc (National Grid Calculator). The program is small and free to use. Simply enter in the coordinates you want to convert, press "Enter," and the program will spit out the resulting national grid coordinates.
