Presumably, you have held a valid FRTOL with the NPPL! The FRTOL is a flight crew licence and provided you can produce evidence of use i.e. Flight Log Book i.a.w. Art 79 then they will renew it free of charge as you have a UK issued Flight Crew Licence
LASORS B1.5
The holder of a UK national flight crew licence with lifetime validity,
who has not exercised the privileges of the FRTOL
for a period exceeding 10 years, will be required to pass
the practical Communications test and theoretical written
examination in order to reactivate the FRTOL.
So the onus is on you to prove usage. A photocopy of your log book should suffice.