The officer's tone was relatively polite, friendly and reasonable and there were no penalties associated with noncompliance. The threats were only subtle: Aside from the fact that it was the campaigner's personal phone number that was used, the officer, having performed some (perfunctory) background research, the officer was able to slip a few pieces of personal information into the conversation. At one point, he joked that phoning the campaigner was "less embarassing than appearing next to you."
Mike from Special Branch was asking the campaigner to be a grass. Whether or not the "chat" would have been about the Olympics or any other issue, he would have tried to get as much information out of the campaigner as possible, putting them in a situation in which it would have been very difficult to avoid endangering themself and their friends.
The campaigner has absolutely no intention of meeting with or "having a chat" about anything with "Mike" or any other policeman. No matter how polite or how threatening they are to you, no matter whether it is in "the cafe of your choosing" or in a prison cell, speaking to the police is *never* a good idea.
If the police phone you, do not agree to meet up with them. Though they might not be putting you in a difficult or frightening situation now, at any moment the friendly veneer can drop and they can begin making life unpleasant for you. Don't give them the opportunity to do this. The only thing that could be gained by meeting with someone like Mike from Special Branch is the satisfaction of your curiosity, and this is not enough to justify the danger in which you would be putting yourself and the people around you. Grasses, informants and infiltrators are tools of the police: being put in this position can only be harmful to you and your comrades.
The best thing to do is to say that you are not interested and get on with your life and activities. Don't let them scare you: if they find out you aren't going to be helpful, then that's that.
It's also good to be open about the fact, making it harder for the police to isolate you as well as more difficult for them to be able to use you. Tell other people, in order to have support from your friends and make the experience easier to handle sensibly. Even posting the fact that you have been approached on Indymedia can be a good idea, so others know that there are dodgy scumbags giving people nasty phone calls asking them to do disgusting things :-)
For more information on being contacted by the police: www.freebeagles.org/articles/grass.html