Flexible hours not agreed: ask the expert

I’m on the last month of maternity leave. I’ve requested my employer to  let me use my holidays for this year to reduce my working days from 5 to 4 until I run out of accrued holidays. I’ve also requested that once my accrued holidays are used up, I’d like to reduce my working days from 5 to 4 on a permanent basis. My employer has agreed to my first request of using the accrued holidays, but on the second request it said that in around 5 months my working hours can be reviewed. I understand that my department is highly likely to go through a restructure in a couple of months. I’d have liked to have agreed to permanent terms from the outset, as I feel that I may lose out on my rights to flexible working if I accept returning to work without reaching a final decision on cutting my working hours. Please could you advise.