Last year, just before dd was born, ds and I witnessed an affray in the street on our way home from nursery. When the yellow incident boards went up, I contacted the police, even though I did not witness the serious racially-motivated assault that had previously taken place on board a bus. The youths in question were pretty scary.
I made a statement and more or less forgot all about it until I got a letter asking me to attend court as a witness.
I did this last week. The youth court where the trial took place has a brilliant Witness Service (pleasant, secure room for victims and prosecution witnesses to sit, volunteer advisers about procedure etc) and although I had to attend two days and hang around a lot, I'm really glad I did it because:
1 - two of the little thugs who beat a middle-aged black guy up got custodial sentences
2 - he is still very traumatised but grateful to the handful of witnesses who actually agreed to testify
3 - The police in the area have been after this gang for ages and were really chuffed to get a result
4 - lots of people on the bus refused to testify. It was down to the bus driver, the victim, me and one other witness (a minor, not ds!).
My experience of the justice system (never even been a juror) was largely positive, if a bit daunting.
If you are ever in two minds about giving a statement to the police after witnessing an incident, I hope this might help you make up your mind. The police and the victim were SO grateful.