The 1964 Civil Service Commission documentation I have here* includes the following:
The cadet's training will take four years to complete and will be divided into five phases of varying lengths. Three of these phases will begin with a course at the School of Air Traffic Control, Hurn, and examinations will be held in each phase.
At the end of the for years' training, a viva voce examination and practical assessment will finally determine whether the cadet is suitable for promotion to Air Traffic Control Officer.
Cadets may be dismissed at any time during their training if it becomes clear that they are not likely to make successful Air Traffic Control Officers.
The very last para states:
Air Traffic Control Offcer Cadets will be called upon to fly in the course of their training, and no additional allowance will be payable for this.
The contrast with RAF ATCO training at the time is stark! Sixteen weeks on the initial Joint Course, and IIRC another six on the GCA Course ... and away you go into your Unit's OJT system
* Min of Av ATCO Cadets Open Competition 1964