Regulation 13(6) requires contractors to prevent unauthorised access to sites, and regulation 27(2) requires sites to be identified with signs and/or fenced off. Isn’t this the same thing?

No. Regulation 13(6) requires the contractor to take reasonable steps to prevent unauthorised access to the site. This could be access by members of the public, site workers, visitors, or delivery drivers. For instance, the contractor may wish to control access to the site, and limit it to those who have received a site induction. Access may be controlled by a gate, a security guard, or turnstile system. A banksman may be used to control vehicles arriving on site.

Regulation 27(2) deals with circumstances where there are risks to health and safety on the site and it is necessary to use signage around the perimeter, or fence it off completely if the risks warrant this. For instance, painting work in an occupied office block may just be taped off with a warning sign. Pavement works in the street might have temporary barriers in place, but a larger construction site with greater hazards may require a hoarding or secure fencing.