If you earned NCD whilst you were insured as an additional driver on someone else's car, we would still be able to use this apply discounts to your policy.
In this case, we would need to see:
Proof of NCD in the main policy holder's name
Proof that you were insured as the additional driver for the number of years NCD was earned.
For example if you are declaring 5 years NCD and earned this as an additional driver on your partner's car, we would need to see your partner's proof of 5 years NCD and then proof that you were insured as the additional on that policy for 5 years.