18. Where, on or after 1st April 2004, a Health Board enters into a GMS contract pursuant to article 3 or 4 with a person who, immediately before the coming into force of that GMS contract, is a party to a default contract with that Health Board, that GMS contract must require the contractor to provide in core hours to its registered patients and persons accepted by it as temporary residents such of the additional services as were required to be provided under the default contract, except to the extent that, prior to the signing of the GMS contract, the Health Board which is a prospective party to that contract has accepted in writing a written request from the contractor that the GMS contract should not require the GMS contractor to provide all or any of those additional services.