Yes, you can write to your employer at any time saying that you wish to regain your right to work not more than a 48-hour average week.

When you signed an opt-out, it may have contained a clause saying how long you would have to wait before the rules applied again, if you chose to reverse your opt-out. This waiting time cannot be more than three months.

If your opt-out agreement did not include this kind of clause, you only have to wait seven days before working time rules apply again.