dvla green slip

[GEDZ]

i recently bought a car(private) we both filled out the v5 form his details my details and he gave me the green slip, next day i went to the post office with green slip to renew the road tax filled out the necessary form handed over the cash and green slip and all went ahead looking good, a week later i received a new v5 with all my new details on it all looking good, two weeks later i get a letter from the dvla saying my green slip is invalid and to send them the updated one or fork out £25 for a new one, so it looks like the seller has given me the wrong documents, i would of thought being that the seller has given me the wrong documents he would be at fault has he should check (by law?) that he is giving me the correct documentation so he should pay the £25 fee has i am not a expert on documents, and it does not state nothing on the dvla letter if i will be braking any laws has i am not a expert on the law.