Yes. If the employee has been employed by you for at least 26 weeks (including her maternity leave), she has the right to request a change in her working hours.
You are obliged to give the employee’s request careful consideration. Whilst you should be flexible in your approach, it is up to you if you grant it or not, but if you decide not to, you must provide the employee with good business reasons for your decision.
If you turn down a request you must be able to demonstrate that your reasons are objectively justified and be aware that the employee could raise a claim against you.