"Manchester Police were unwilling..?"
They did not appear to want to make the crowd step to the side. The Manchester police were there, maybe six to eight of them. They only person I saw them ask to move on was a guy with a pit bull who was trying to pick a fight with anyone, but they did not ask the people to stand aside to allow anyone to bet able to get to the door of the campaign headquarters. That however may have been a moot point since there was no way they could have vetted everyone in the crowd.
See http://www.concordmonitor.com/article/303787/trouble-at-hq?CSAuthResp=1326211864%3Ap9lqn2tr69rju675k1j0cmrgh5%3ACSUserId%7CCSGroupId%3Aapproved%3A02D65EB4063CDFE7F8A1338063711CEE&CSUserId=94&CSGroupId=1 where the Manchester police advised that the crowd created an unsafe condition.