From LASORS Section B, 1.5:
Where a FRTOL is issued in conjunction with another UK issued flight crew licence the validity period of the FRTOL will be the same as the flight crew licence to which it is associated. If the associated flight crew licence is allowed to expire, the FRTOL shall be valid for a period not greater than 10 years from the last renewal/revalidation of the flight crew licence.
Where a FRTOL is issued/renewed in conjunction with a UK flight crew licence, no charge is made for the issue of a FRTOL.