I have just come across this experiment in the US on what people would do if they observed racism in action. I am not sure when it was conducted, but I found it illustrated what psychologist call "bystander" behaviour which allows injustices to continue simply because many/most people fail to intervene. Injustice often continues simply because of silence by the majority - and especially the silence of those who are not part of the "discriminated" group. The message of the experiment is: if you observe racism, discrimination or injustice don't remain silent - speak out and challenge!
It would be interesting to know if there have been similar experiments in the UK (e.g. around Stop and Search by the Police for example). If you know of any such experiment, please let me know. Email firstname.lastname@example.org