OneEyedJoe- I keep thinking about this, ie a sign that cant be removed by the dubs, or get asked to "move on". I have a number of big white vans that could be adorned with a vinyl sign of some kind, with slogans like the UN farce. Parked up outside an assembly hall, parking paid for the whole day. The dubs cant get you to move on, and signs on vans dont need planning permission.
Newcastle city hall anyone? Would work well there!