Massive caveat: IANAL - so get thee over to
PePiPoo and don't take this post as "gospel".
Take a careful look at the wording on the form. If the form merely requires you to fill in the details on the reverse then get a photocopy of the form and get the copy authenticated (signed by someone independent to say it is a true copy). Then send it back completed as requested (i.e. blank - they didn't actually ask for anything, so you'll have filled in all the details they requested on the reverse of the form). When they take you to court, you've met your S172 obligations by provided the information they requested and I suspect the scammers won't have a leg to stand on. I'm not sure, though, whether they can issue another S172 request having served the original NIP within 14 days.
If, however, the wording is such that it requires you to identify the driver, I suspect your best chance would be the so-called PACE witness statement - again, see PePiPoo for details.
HTH,
(Edited to make it clear that if they've asked you supply the details specified by questions on the back of the form, the back of the form is blank, and you return it blank, they haven't actually asked you for anything and I suspect you've met your obligations under S172).