A consent given under this Part by the Secretary of State to a regional development agency—
(a)may be given unconditionally or subject to conditions,
(b)may be given in relation to a particular case or in relation to such descriptions of case as may be specified in the consent, and
(c)except in relation to anything already done or agreed to be done on the authority of the consent, may be varied or revoked by a notice given by the Secretary of State to the agency.