ACoP CDM Regs 2007 paragraph 150 (g) (j)( Principal contractors duty) “Take reasonable steps to prevent unauthorised access to site"… So yes, you secure your part of the site. No it is not your responsibility to secure the main site, that duty lies with your client. Obviously the CDM-C should assist the client and ensure that they are made aware of the duties

Mick the Acops states "take reasonable steps" Paragraph 171 deals with access on site by unauthorised persons it states: The boundary of the CDM site should be physically defined (ie suitable fencing.) with special consideration to (A) rights of ways cross sites. (B) sites in or next to other work areas. (C) new house developments where some are already occupied. (D) where there are children or other vulnerable persons nearby. Reg 27 (2) of the CDM 2007 clearly states that "Where necessary in the interests of health and safety a construction site shall, so far as reasonably practicable and in accordance with the level of risk posed either - (A) have its perimeter identified by suitable signs and be so arranged that its extent is readily identifiable; or (B) be fenced off, or both. Reg 25 CDM 2007 States "Every person at work under the control of another person shall report to that person any defect which he is aware may endanger the health and safety of himself or another person". So inform the Client of the issue. Ensure your site is secure or if that is impossible ensure any danger areas are fenced and secured (As on roadwork's it would be impossible to secure the whole road so plant is locked away or made safe and any excavations securely fenced). That would then be your duty fulfilled (reasonable steps) I believe you should explain this to your client that they also have a duty of care and perhaps offer to organise and supply guards at their expense. I will also point out that the owner of the premises has a specific duty of care under the HASAW 1974 section 3 (Duties to others) that they do not expose themselves or others who may be affected by their activities to risks, so far as is reasonably practicable. Others include the general public; contractors; visitors;trespassers. Controllers of premises have similar duties under sect 4.