Footage from BBC News coverage of the TUC protest day shows unusual behaviour from a particular "protester" with a black hood and white trainers. The man appears to show something to a uniformed policeman and then crosses police lines in the middle of a fight.
It is obviously now important to find out if there is footage elsewhere of this protester being violent.
Hold up... here we are. This guy may be the same one that rampaged through HSBC. Starts 1.13, he was outside before then with what looked to be a beer can.
Point against this theory: the trousers of the man in the first video look lighter - perhaps blue, whereas this guy's are black. That doesn't rule him out, of course. Having been witnessed on video he may have gone to a shop and changed them. He could claim back the cost on expenses.
Here are some stills
Entering HSBC intent on causing some criminal damage
He didn't like the camera
Blending into the crowd