So, as mentioned particularly in the comments of the last dispatch I think we've probably got bigger problems than perverted priests, and I feel a little bit bad piling on the Catholic Church, particularly since I know and love some really lovely devout Catholics.
But damned if I didn't go anyway (more in the spirit of Here's This Cool Event Where I Live and I'd Really Rather Not Be Personally Paying For A State Visit From This Guy rather than God Save Us From The Evil Of Holy Men), and I had a cracking time. Here's a brief rundown.
I developed a strategy which involved gearing up and running through the park to Hyde Park Corner in time for the start of the march. I was a bit late, and ended up chasing them down Piccadilly chasing perhaps as many as 20,000 people, as it turned out. This was rather more than the few I hundred I was kind of expecting.
I totally randomly also had the wonderful experience probably number one on the list of reasons one lives in a place like London of randomly encountering, and being adopted by, a really amazing little group of people. (Including an entirely charming and erudite clinical psychoanalyst married to an actor-turned-novelist, and a documentary filmmaker dressed as a naughty Pope.)
I neglected to bring a camera but here's a photo set by someone else that stresses a bit the freaks that came out, but generally seems to capture it. (Mouseover for player controls and thumbnails.)
Hey! I just noticed I'm in one of the ones above! That's my left ear, above the 'Religion is Silly' sticker on my FDNY muscle shirt. Beside me is the naughty pope/documentary filmmaker.

Finally ended up in Whitehall across from Downing Street with, actually, the Cenotaph right between us and the speaker. However, I dashed forward when I heard Dawkins go on, and managed to see (and, more or less, hear) him while up on tip-toe.
I ran home.
Oh! today was also my seven-year anniversary in London. Well spent.