Things I have learned as a possible witness in the last Critical Mass trial; in no particular order:  

• Allowing a police (liaison) officer to talk to you puts you in real risk of prosecution (after being kettled for hours, arrested, taken to distant police stations, made to wait for hours again to be processed, and eventually let out with your bike (or not!) in the middle of the next day). What the prosecution needed to prove was that the defendants knew about some obscure conditions that the police chose not to disclose until the event itself. Having a piece of paper with those conditions given by police in your pockets, or having had a chat with a police (liaison?) officer, was the proof the prosecution needed. So, the learning is, allowing a police (liaison) officer to talk to you puts you in real risk of prosecution.
• Lawyers are not necessarily efficient in getting in touch with the witness they need. I give two examples: One of the defendants asked me to be his witness. I told him to give my details to his lawyers. I expected a call or an email for a few months, while they were building up his case, presumably. Then the hearings started, and, seeing that I had not received any call or email from the lawyers, I called the lawyers myself. Only then I finally got to see them and present them my evidence. Then, once in court, a lawyer I had not seen before, approached me to ask me if I would be happy to be a witness for his defendant. I found a bit disheartening that, after the loads of statements that Green and Black Cross handed the lawyers, they were still short of witnesses two days before the end of the trial.
• As a potential witness, you may not be required to be an actual witness. I took the day off, put on some of my best clothes, waited around for about two hours and then was told that it was a tactical decision not to call me as a witness. No criticism there; it may be totally understandable not to call a witness that will say things already mentioned by previous witnesses, or whatever other reason may be to not call some of us. And the advantage was that I could sit in the public gallery to witness the rest of the day’s proceedings.
• In prosecution jargon, ‘not enough evidence’ seems to mean ‘not enough evidence to counter-act the overwhelming evidence presented by the defence’. My line of reasoning here is that, for the two first defendants to have their charges dropped, the prosecution thought there was enough evidence to press charges (and that is how it should work). But that was before presented with the evidence that these defendants and their witnesses were able to provide. After this new evidence in favour of the defendants, then it was that the prosecution’s evidence was not enough.