If you have been employed by the same employer for at least 26 weeks (including your maternity leave), you will have the right to request a change in your working hours. The right to request is a formal process where your employer has to hold meetings with you and give you a decision in writing.
If you can, it will help the process if you try to start discussions about your working hours well before you are due back from maternity leave.
If you and your employer are unable to agree and you need to follow the formal process it can take up to 14 weeks to consider all the options that might be available for you. These options include going part time, working from home (at least for some of the time), job sharing options, compressed hours (where you work your normal number of hours but over fewer days), flexi-time options, annualised hours (where you have the flexibility to change when you work from week to week so long as you work the contracted hours over the next year), and staggered hours (where you agree different start and finish times with your employer).