How can I change some of the basic terms of my contract of employment?

A contract of employment is a legal agreement between the employer and the employee. It contains terms, either express or implied, which you cannot lawfully change or vary without your employer's further agreement.

If you wish to change some of the terms you should discuss the matter with your employer, perhaps with the help of a trade union representative. There is no legal obligation on your employer to vary the terms however, except in limited cases, for example, where an adjustment of hours of work is deemed a necessary reasonable adjustment for a disabled worker.