Another way is to use Google Earth (or Maps). Stick a placemark in and use
this website to convert the co-ordinates. Nearby.org.uk also has a neat interactive overlay for Google Earth which draws 1 km grid squares.
The OS provide a downloadable tool for converting Lat/Long, UTM etc. to and from the National Grid. Useful for large quantities of data.
In the past I've created files for Google Earth which overlay my detailed recording grid on to the aerial photography. Now life is simpler since I've been given a GPS: it dramatically shortens the time needed to record if you want precise gridrefs.
Both Magic and EarthTools are useful recommendations which I wasn't aware of. Notice though that the contours of Snowdon shown on Earth Tools have a big hole at the col between Crib Goch and Crig-y-Ddysgl: this is because the contour data are sourced from the ill-fated NASA Challenger Mission.
posch