Hm. http://climatecamp.org.uk/g20/legal suggests writing to your MP - obviously if you weren't involved then it's more about 'here is this report / information on Ian Tomlinson, I am horrified & what are you going to do about it' than it is 'I was batoned by a police officer, what are you going to do about it'. But I think it's still useful - even *more* so when non-directly-involved people complain, because it shows that it's not just Those Activists who are aware & upset.

More generally: there's lots of suggestions for getting involved at that website!

NB: I wasn't personally at G20, due to WrongContinentError, although I was at Climate Camp last summer, which also featured exciting riot police action. (and the government passing police info onto E.On! How nice of them. http://www.guardian.co.uk/uk/2009/apr/20/police-intelligence-e-on-berr )