How do we know there aren't more cases where police have used excessive force?
When Mr Tomlinson was first injured, the initial report from the police suggested that it was a member of the public who had caused his injuries. It was only after a man, an American I think, released a film of the incident to the Guardian that the police admitted it was one of their own officers who had pushed Mr Tomlinson over. It was only then that the IPCC opened a full investigation (they'd previouslsy refused to). They proceeded to get a very dodgy pathologist to carry out the post mortem examination. He concluded that Mr Tomlinson had died of a heart attack (he was I think awaiting a disciplinary hearing and has subsequently been de-registered). Three further pathologists disaagreed with this finding and stated that Mr Tomlinson's death had been caused by internal bleeding. It was subsequently revealed that the police officer had some years earlier been involved in a "road rage" incident but before the matter could be properly investigated, he retired (on full pension) on medical grounds. He then worked for the police as a civilian member of staff and later re-applied to the force and was taken on as a police constable, where twelve more complaints of improper use of force were lodged against him, including one brought by a fellow officer and another by a member of the public who said he'd seen the police officer assault someone.
In every profession there will always be people who are inadequate to the task, who abuse their position or who are temperamentally unsuited. I think the more worrying issue in this matter is that there appears to have been an attempt by other people in authority to cover up what had happened. It's therefore hardly surprising that some people don't automatically trust the judicial system on every occasion.
On the subjects of demonstrations, in my view, it is sometimes justifiable for people to demonstrate and even to commit acts of civil disobedience. Many people now admire those suffragettes who risked life and limb, and were force fed in prison, in an effort to obtain votes for women. At the time they were widely disparaged (by women also).
