8. Subject to the provisions of this Act, an authorised person shall be entitled to remain in possession of a derelict tank for such period as the Collector may determine after consideration of the time necessary to compensate the authorised person for the cost incurred in the improvements carried out together with interest thereon at the rate of ¹[fifteen] per centum per annum, but not exceeding twenty years from the date on which he takes possession thereof under section 5 or section 6: Provided that the Collector, upon application made or of his own motion, after considering the views of the authorised person, may at any time reduce the period of possession determined by him under this section or extend such period subject to the maximum limit of twenty years referred to in this section, according as it appears that the costs incurred-(a) by the Collector in carrying out the purposes of this Act in respect of the tank, and(b) by the authorised person in carrying out the required improvements in the tank,will be recovered together with the interest thereon at the rate of ²[fifteen] per centum per annum within a period shorter than, or will not be recovered together with such interest within the period of possession so determined.