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, once you had reversed your opt out. This waiting time cannot be more than 3 months.
If your opt-out agreement did not include this kind of clause, then you only have to wait 7 days before working time rules apply again.
