There is 10 stopper present
I had a few problems with pulling off the disappearing trick.
This bridge infamously moves/bounces around quite a bit when a large amount of people walk across it, +30 second exposures were suffering from motion blur due to the movement of the bridge. Found 15 seconds to be a not too bad compromise.
Secondly, too many people were lingering on the bridge, stopping and standing still to look at the view. Quite annoying at the time.
I had to be quite quick, as it's not a great place to have a tripod extended, quite a narrow bridge really. But I agree with mattyg, I think I've come away with a more interesting shot by having the people there.
I was stopped quite quickly by Canary Wharf security and asked what I was doing, had the Lowepro AW400 and a tripod strapped to my back. I told the security guard that I was taking photographs for private/hobby use. He was polite enough and told me that's fine. But asked me not to photograph security cameras or block any entrances or exits. I've heard some instances where the security there can get quite funny.