John,
My '57 Oval had been in the country unregistered for about four years before I started the registration process. Like you, I had no papers for it, not even a sale invoice, as I bought the car as a cash purchase. My own apathy over bureaucracy, and the desire to get on with the restoration meant that I only recently got around to applying for an age-related number.
I spoke to HMRC by telephone, and they advised me to include a covering letter explaining how I came about the car with the NOVA declaration, but they also warned me that I might still have been liable for import duty. Happliy they accepted my explanation, and I didn't have to stump up any money. I was then able to apply to the DVLA for a number. When I did this, I included the HMRC's letter giving me the go ahead to obtain the number, however I believe HMRC and DVLA share information anyway, so this might have been uneccessary. It's better to send anything and everything you can think of.
If you're a member of the owners club for your car, it might be prudent to speak with them. Quite a number of enthusiasts clubs have signed up with the DVLA, and if they are, they should be able to support your application.
In the 'good old days', the process was, of course, much easier, and when I was secretary of the HVWC, I helped several Club members obtain age-related numbers for their cars.
I hope you're successful.
Gavin.