This precedent has been authored by Dr. Gordon Hughes, Principal, Davies Collison Cave Law.
Generally, a customer will seek the right to approve any sub-contractors which a service provider proposes to use. Unless the contract states to the contrary, the contractor will remain responsible for the actions of the sub-contractor, and it is best to remove any ambiguity in this regard. Depending upon the sensitivity of the project, a customer may seek a right to remove a sub-contractor at any time.
Sub-contractors are sometimes restrained for a period of time by the terms of their sub-contract from subsequently working direct for the customer. A customer may seek to circumvent this restraint by obtaining an express acknowledgment from the prime contractor that it may (at least if the head agreement is terminated by the customer for breach) directly engage an existing sub-contractor in order to ensure continuity of service.