Do employers have to keep records of the hours worked by those of their workers who have chosen to opt out of the average 48-hour week?
No, employers do not have to keep records of the hours worked by opted-out workers. Regulation 4(2) of the Working Time Regulations 1998 (SI 1998/1833) provides that employers are required to maintain up-to-date records of all workers who have signed an opt-out agreement under which they agree to disapply the 48-hour maximum working week. However, this could simply be a record of the relevant workers' names, together with copies of the signed opt-out agreements. It is no longer necessary for employers to maintain records of the hours those workers actually work.
On the other hand, where workers have not opted out, reg.9 states that employers are required to keep for two years records that are adequate to show that they have complied with the provisions on maximum weekly working time, although the Regulations do not specify the format in which such records should be kept.
In Federación de Servicios de Comisiones Obreras (CCOO) v Deutsche Bank SAE  IRLR 753 ECJ, the European Court of Justice (ECJ) held that employers are required to keep records of the daily hours actually worked by workers. The ECJ concluded that EU law means that national laws must require employers to set up a system enabling them to measure "the duration of time worked each day by each worker". Amendments to the Regulations would be required to meet the standard of record-keeping set out in this ruling. It is unclear if and when these legislative amendments will be made.