Goon

The answer to Police behaving themselves at demonstrations cannot be trial by camera phone.
Like all service providors they need to be kept honest by anonymous observation. The mystery shopper approach used by many in the private retail sector would work well.

The various forces should recruit and train people who are neither pro or anti police to attend demonstrations in pairs to monitor behaviour. The knowledge that the person you are interacting with could be monitoring you will improve their behaviour and weed out those who are too aggressive.

Mystery shoppers are paid but mystery monitors would do it for expenses (no not second homes type expenses!). Lets get the ball rolling I'll be first. Who will be my mystery monitoring buddy?