Turns out their "10,000" was actually 12,000 and was actually an estimate of the maximum number of people in Trafalgar Square at any one time, not the size of the demo at all. And it was estimated by one person, based on the licencing capacity of the square. No sampling. No computer analysis of aerial photos. No thought that the licencing capacity might be based on health and safety rules, number of exits, not the number of people who will actually fit in the square.

The story is at  http://www.bbc.co.uk/blogs/opensecrets/2007/08/how_many_there.html