i have had similar interactions with the dvla. my 6711 has a number plate, i contacted the original vendor and they confirmed reg, eng and chassis numbers and the original buyer , got it inspected by a third party who says all is good, but the dvla say that the details dont match the ones they have on file so i will have to re register and have an age related plate!
if anyone knows how to go over the jobsworths head let us know.