Sorry to read of your issue Ian, Though I suspect it is perhaps a foul up in the data bank. I`m guessing you used the DVLA system to check MOT history for the reg to see what comes up..?
I have known where the same reg was apparently issued to two vehicles many years ago, one of which was not used on the road for most of it`s life, and only when subjected to a HGV inspection given the possibility of road use, did the issue come to light... Unfortunately I do not remember the way it was resolved.. sorry.
I suspect it can be sorted it though how much time that might take these days..?