Not geocaching, but I'm trying to produce a map-driven website using Open Streetmap (OSM) tiles. Because distances between points are fundamental to my application, I'm using coordinates based on the OS grid internally because OSM tiles are based on angle of lat/lon rather than distance across the ground. This means that I need to know the OS grid ref of each corner of each of the OSM tiles I'm using.

I have GSAK, which has done the trick for all but the bottom row of my map. While GSAK gives me the OS grid ref for the top of those tiles, it won't for some of the bottom edge. For example, I need the OS grid ref for (49.837982, -2.109375) to within a few metres.

Can someone recommend a (preferably free) WSG84 to OS grid converter that can do this?

TIA,

Geoff