if the address you put down was the address the buyer gave you then as far as you knew it was correct, do we have to do the DVLA's job when we sell cars now?
It could mean that the person you sold the car to has given you an address that is incorrect or does not even exist . I do not know of any rule that you have to check that someone has given you a correct address . Do you know the address and name? if so look it up by checking the postcode on the Royal Mail postcode checker site it will tell you if it is a real address or not. If it turns out to be a false address i do not see what you can do other than tell the DVLA what you were told again.
i wonder if the reason they have contacted you is because the person that bought the car from you has sold it straight on to someone else and the second person has then regsiteredf as the new keeper and the dvla has cross referenced your v5 with the new regsitered keeper details and it does not match, in which case, the middle man could be tax dodging