Just to complete this thread -

After 2 years (mainly delayed by me being lazy TBH) this has been resolved. DVLA initially replied and said "All is well. Registration and V5 are correct" with no other explanation. I wrote to them again and asked for clarification on how the situation came about, if i ever came to sell it I'm sure someone would ask the questions I should have asked!!! After a few weeks I got a letter saying they had looked into it again and had assigned the original correct age related plate for the vehicle and issued a new V5 with the correct registration year of 1980, 1 year older than originally thought so it turns out both the V5 and the registration number were incorrect. Again no explanation of how they came to this conclusion.

Anyway, happy with result in the end.... maybe this will help someone else in future.

I need to go through the same thing with mine when it is back on the road, it went to NI in 78 but when it was imported back in 86/87 it was declared new in 81 so got an X reg rather than a T, its annoying me so I will sort it eventually.
Glad to know it can be done, without too much pain.