Where the duty sits
The employer or the project supervisor is responsible for having the plan prepared. That is a separate question from who physically writes it: delegating the writing to a consultant does not delegate the duty.
The coordinator's role
- Preparation-stage coordinator: ensures the plan is prepared.
- Construction-stage coordinator: takes progress and changes into account and makes or arranges the necessary adjustments in the plan and the file; also puts in place arrangements to keep unauthorised people out of the construction area.
Why not the site manager
In practice the plan usually ends up on the site manager's desk. That is a division of labour, not the location of the legal duty. At an inspection the counterpart is whoever is responsible for having the plan prepared.
A detail from the measurement
The regulation never uses the word for "site office"; it says "construction area" from beginning to end. It reads like a detail but it fixes the scope: the construction area is the whole area where the work is carried out, not only the footprint of the building.
Is a subcontractor's plan enough?
A subcontractor's own method statement does not replace the plan for the construction area. The plan covers every party on site and the interaction between them; a document describing one subcontractor's work cannot.