Gibbo :o)

First things to know are:

is there separate entry/walkway for pedestrians

is the barrier electic with warning alarm, or is it manually raised

if electric is there a detector or whatever to prevent barrier coming down on vehicle, and would it detect a pedestrian who was trying to walk under the raised barrier

Presumably all traffic continues to enter and exit in the area where the new Gatehouse etc. is being constructed?
Temporary measures would be necessarily quite limited, with a couple of signs (one for each traffic direction) saying all vehicles stop (at the line, in the hatched area - whatever you choose to temporarily paint on the ground as a temporary measure) here, and the chap in the hi-viz and the raised hand sensibly positioned so that he can avoid or at least step out of the way of the dozy driver who fails to stop?

Yes, you could put up some lightweight temporary barrier, but that will get tedious - especially if it's windy!

any pedestrian access should (temporarily) be as far away as practicable from the construction 'site'.

The reason that security is required should be considered when determining what measures to put in place.

If they are moving relatively slowly it would appear not to be high risk, but it would be useful to know what the employer wants the security system to achieve.

thanks to all respondents,the client now realises the potential safety implications and found a company that fitted the barriers over the weekend.
amazingly before our meeting,they were looking at 6 week rollout !!!!