Notifications and other communications from the contractor can be given by anybody with its apparent authority. The contractor is a corporate body and they therefore has to act through its employees and others who represent it. The same applies to you as the employer – you can also allow anybody with apparent authority to carry out your actions under the contract. However, the project manager and supervisor are named individuals and are required to carry out the actions required of them personally, unless they have delegated such actions in accordance with the contract.
It is irrelevant as to whether or not the people communicating on behalf of the contractor are named as a key person, as the contract makes no such requirement for them to be so. In any event such a requirement would be impractical and ill advised. For example, an early warning is required to be notified ‘as soon as’ the contractor is aware of a matter listed under clause 16.1. The person who firsts becomes aware will often be the site manager and it is therefore good management to allow him or her to notify accordingly.
Of course it is good management to discuss and agree with the contractor who will usually be making the various communications required by the contract. But that will not preclude the contractor from using other people for communications, and it would not render those communications invalid.