A quick big-up for a new dynamic overlay for Google Earth that adds the OS national gridlines. The link is obtainable via this post. It works well for me, as you zoom in the grid squares are gradually replaced until you are viewing at 1km scale. I think that this could be a useful demonstration for students.
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Works well for me too! Apart from the obvious use for teaching GRs, it makes linking OS maps and aerial photos much easier. I have just checked it out for Dundee and it is really easy to cross reference individual buildings on the map and the GE image.