... you could find yourself a friendly hacker, hack into the police computers (quite easy so I am 'told' he he!), find the address of said copper and take the matter up with him personally - rather than waste your time asking the state to do it for you (which they won't)

Seriously, if you want to lodge a complaint, get the injuries verified by a professional ASAP. I'm not that sure about the complaints procedure, but I'm pretty sure you can pick up leaflets at most stations (not that I think it will do much good, IMHO), if the complaint does nothing, there's some info on suing the police at schnews:
 http://www.schnews.org.uk/diyguide/suingthepolice.htm
I was in the middle of the crowd, so I didn't see any brutality 1st hand, tho I'm told they were well out of order.
5 of us sat in front of the US Embassy earlier (must've been about 3pm) sat trying to reason with the cops for a few minutes (with support from passers by) before they manhandled us to the 'designated protesting area' (what a bunch of crap).

On another note, big thanks to rhythms of resistance for the music =)

this is becoming prevalent now. as a keen amateur snapper, ive had threats to have my camera broken by underground managers, by park -keepers and shop owners.
quite recently of course, our own dear deputy prime minister beat up a protester (who was able to land one back im glad to say) and the rest of the crew at No. 10 gloated in public.
in short, we are living in an age where casual violence by frustrated officialdom will become ever more prevalent. take a helmet to demos as a basic precaution, for even if YOU intend to be entirely peaceful about things, the thugs and coppers- friends MAY NOT BE

We turned up pretty early on and saw the first poeple being picked up, it didn't look like there was any necessity to cause pain - there was enough coppers to pick anyone up who didn't want to. And it just created an angry vibe...

Anyway here's a video of the practice being used. (divx avi)

ok, wot u do is this...

Get your injuries photographed, and examined by a doctor. Get doctor to sign a statement to the effect that your injuries, in his/her opinion are consistant with being attacked in the way you describe.

Then, get witnesses. Make a request on indimedia if you have to. Get them to write and sign statements, and make sure they are willing to testify.

Collect any photographic evidence. Get pix taken, and importantly suss out if there was any cctv cameras rolling, or if you were bein filmed by cops. Mark Thomas's website has a whole lot of info on how to request this stuff, its your right and can be used as evidence against the cops.

Then, get a solicitor. Go to Moss & Co, Bindmans, or Bhatt Murphy - you should find em in yellow pages. They are used to dealing with this stuff.

Then, make a police complaint if you want to (go to the mets own website for info) but wot I reccomend is that you sue their sorry, big fat asses!

Good luck!