The previous owner of my car sent a letter to the DVLA asking for any information and documents they held on their system. The original import documents were one of the documents sent out to him.
I got all mine from the DVLA... There's a request form you can get from DVLA online, complete it and the they will send you all the paperwork they have on the car.
I got a right wedge, shipping docs etc (granted everything was in Japanese but hey ho, good to have)
Just been looking at the export certificate of my DC5 and found out the the registration of my car in Japan was 大阪 399 㡠9663 which translated is Osaka 399 Chi 9663. Sadly the owner name and address comes up as a Japanese trading company so I guess a dealer.
So cool. Wish I could get a Japanese plate made up now.