As a nanny your employer has a legal obligation to provide you with a contract of employment within 2 months of your date of starting, this should detail the terms and conditions of your employment and you should ensure that this includes at least some of the following:
Your name and job title
Working days and hours
Salary
Holiday entitlement
Duties and responsibilities
Statutory payments (SMP, SSP)
Workplace pensions
Notice period
Disciplinary and Grievance procedures
You should always check and read the contract fully and understand the details and anything you are unsure of or you believe is not included should then be followed up with the employer before you sign and date.