Thank you for your reply.
Employers can rely on a flexibility clause, even if it makes changes that are detrimental to the interests of the employee. However, tribunals will use the existence of implied obligations in the employment contract to restrict the way in which a flexibility clause can be operated.
The leading case is United Bank v Akhtar  IRLR 507.
The employer should give reasonable notice of any move, notwithstanding the scope of the mobility clause.
- The employer would not exercise its discretion to provide relocation expenses in a way which made the employee's performance of his obligations under the employment contract impossible.
- The employer would not act in such a way so as to damage the relationship of trust and confidence which exists between employer and employee.
If the change in shifts is breaching the duty of trust and confidence then they should not act in such a way and allow you to continue working as you are.
You should definatley ask your employer for a written contract. The employer should have issued you with a contract at the beginning of your employment. You are able to sue your employer for not providing you with a contract of employment.
First I would suggest you ask for a writted contract if non is given then your employment. If there is a dispute the court will decide what the actual terms are by looking at all the other documents and evidence. The terms will then usually be what is in the employee's "statement" given to them by the employer.
Other sources of terms that a court may say are part of the contract are as follows:
a) The letter the employer sent offering the job (if one was sent). These letters often contain important information about the terms if the job is accepted.
b) Terms agreed between the employer and any Trade Union that represents the workforce. This includes local and national agreements made between employers and Trade Unions.
c) Any terms stated in an advertisement for the job.
d) The employer's works rules and staff handbook, this includes rules placed on the staff noticeboard by the employer.
e) Spoken agreements between the employer and all the employees or individual employees
If you belive that the shifts given in your oral contract are the ones which you have been doing for the past 10 year or more then your employer is not able to change your contract without your agreement.
I hope this answers your question, if so kindly rate my answer positively so I can get credited for my time.