Can I offer a few bits of info on this:-
1) aside from the (rarely used by most of us to be honest) terrorism powers, neither police officers or PCSOs have any statutory power to enforce crime scene/accident cordons.
2) however, as stated above, we (police officers and PCSOs) CAN enforce cordons under Common Law, which may include physically preventing someone trying to get past you. See pg 51 in ACPO document below:
http://www.acpo.police.uk/asp/policies/ ... 202009.pdf
3) failure to adhere to police/PCSO direction not to enter cordon may well result in arrest (by a police officer only), for obstructing police under s89 Police Act or the obstruct PCSO offence, Police Reform Act. However as both these offences are Summary only, s24A Any Person powers of arrest for PCSOs or anyone else DON'T apply. As this is not a "Relevant Offence", no Detention Power is available either.
4) Use of force to secure a cordon, using s3(1) Criminal Law Act ("A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large." )
may be a valid tactical option for police officer or PCSO (but for PCSOs, ONLY with regard to "prevent crime" part). This removes issue of "assault" I'd suggest. The crime would not be crossing a cordon line, but "obstructing PC" or "Designated Person".
5) HOWEVER...PCSO deployment policy must be adhered to. Regardless of the above point, if your Force stipulates the "hand on the shoulder" as the extent of the force you can use, then unless you have someone trying to enter a burning building or some other immediate threat to life, you'd be struggling to justify any s3(1) use of force beyond your Force policy, merely to enforce a cordon.
Like people have said here, 9 times out of 10, people just accept that we have a job to do and comply. For the 1/10 morons who think otherwise, just summon a PC I'd suggest.