Hmm, let me see:  
22 July 2006, against Israeli bombing of Lebanon:

http://news.bbc.co.uk/1/hi/uk/5203450.stm

"Police estimated that 7,000 people took part in the London rally but 
organisers said more than 20,000 people attended the march from Whitehall 
to Hyde Park, including families and representatives from various faiths. 
No arrests were made during the protest. "


http://news.bbc.co.uk/1/hi/uk/2765041.stm
16 Feb 2003  March against Iraq war

"Police said it was the UK's biggest ever demonstration with at least 
750,000 taking part, although organisers put the figure closer to two 
million. "




23 Jan 2008

From http://news.bbc.co.uk/1/hi/uk/7203839.stm


"Both the Met Police and the Police Federation estimated that 22,500 
officers had joined the march. "


This must be the only time in living history that the Met Police have not 
said the organisers are exaggerating their numbers.



Well I saw you go past and NAAAAH! no way!  I would say 6,000 tops and 
that's pushing it :-P  








PS. now you know what it feels like! 

PPS. sorry, couldn't resist. :-))

PPPS. hope you get your full 2.5% backdated though, and well done for 
protesting over it.