As well as providing written terms (a 'written statement of employment particulars') it’s a good idea if the employer puts the following in writing, so everyone’s rights and responsibilities are clear:

• the job offer
• any benefits, such as private health insurance
• how the employee will be paid (for example, pay will go straight into the employee’s bank account)
• the length of any probationary period
• conditions of any probationary period
• the standards of behaviour it expects from its employees and what happens if these are not met (for example, the employer will report any thefts to the police)