“He is under a duty to detain the prisoner for the required period, neither more nor less, and must calculate the earliest date on which the prisoner can be considered for parole and the date on which he is entitled to be released on licence. Both of these have to be calculated by reference to the period of imprisonment specified in the court order. Once the date for early release is reached the governor has neither the right nor the duty to detain the prisoner any longer and would be acting unlawfully if he were to do so.”