I had a similar issue selling a spanish car. The UK left hand drive specialist promised to do all the new registrations and deal with all the papers.
The car got sold to another brit in spain - but the papers were never done. I got speeding, parking and non-payment of taax fines for 3 years after I sold it until I personally went to Traffico in Alicante with the bill of sale papers and informed them face to face - with my passport, drivers licence etc that I had not owned the cars for three years.
They reported the car as haaving been eexported (I sold it in the UK), they then put out an arrest warrant foir nthe guiy who was driving it around as his insurance papers were now invalid.
I could not get the 700 Euros back in fines I had paid - but he will get arrested andd fine and charged for importing aa car without registering it - which for a car of 1500 Euros value is going to hurt - JOY!
I believe you will have to personally go to a Notary or even go to Traffico in your nearest appropriate branch and report the papers missing and apply for new ones. I could be proven wrong - but in my case - writing to them for 3 years after I'd sold the car and didn't have papers for it cost me 700 Euros in fines. How much do you value getting those papers sorted?