I have updated my OSM-compatible files of the British National Grid (http://www.rogercalvert.me.uk/OSM/GBGrid), by removing 100km grid squares which show nothing useful (such as empty sea), thus reducing the file size by about 35%. The outline can be seen at http://www.rogercalvert.me.uk/OSM/GBGrid/GBGridSquares.bmp.

Contact me if there are any problems.

Regards,

Roger

Roger Calvert