You may need a website to run this, a free website can be found on angelfire.com, though to run the program you do not need a website, you just need to access the ordnance survey website.
Word of caution, when using the program it is easy to create a marker when you click "find", so when entering sites, it is best to click 'create marker' and then select 'edit marker' and then click on find.
The best way to select precise locations is to have googlemaps running at the same time as running the program so that the marker can be precise.
