What steps have you taken to ascertain who was driving?
The duty is upon you to ascertain who was driving.
S172 does not ask if you know offhand who was driving as, of course, you do not.
It demands that you investigate and ascertain who was driving. That is usually possible bearing in mind that this can only have been under 14 days ago.
If you genuinely cannot then there is a defence in saying that you were not able to find out with reasonable diligence but the burden of proof is upon you rather than upon them and you do need to show what steps you have taken.
Can I clarify anything for you?
Is that the first NIP?
Are you the registered keeper?