Impact College Ltd provides private tuition. The college is managed by Jack, who also has responsibility for personnel matters. Fred and Gale are amongst the 60 lecturers currently employed by Impact College Ltd. They have both worked for Impact College for the past six years. Fred has been the staff trade union representative for the past three years and has had several confrontations with Jack as to the working conditions of the college's employees. Gale has been off work twice in the past four years on maternity leave, to Jack's stated annoyance, and is pregnant once again. It has transpired that Fred and Gale are amongst the ten members of staff selected for dismissal by the college and they suspect that Jack has pursued a personal vendetta against them.
【论述题】
Identify what an employer must prove to avoid being found liable for unfair dismissal