If an employee works at or close to the maximum, ask them if they are willing to opt-out from the 48 hour limit.
If not you must take reasonable steps to ensure that working time stays under the limit.
Employees also cannot work for more than 13 hours a day and must either have 24 hours' continuous rest a week or 48 hours a fortnight.
Using a separate job description works best if you want to give a detailed description of the job and so can help bring clarity, particularly for mid-level to senior employees.
For an employee working five days week, this equates to 28 days including bank holidays.
Employers can agree to a greater holiday allowance.
However, you should only reference a separate job description if it will genuinely be created in practice.
Otherwise, it's better just to put a brief explanation in the employment contract.