It is likely that it was last taxed before the sorn came in, so it didnt need sorning but the DVLA said if you didnt sorn it then they would wipe their records of it and you would never get a log book without re registering etc etc, which was of course a load of cobblers. £25 later and a few form filled out. wait a few weeks to see if the registered keeper protests and you should get a log book unless somthing dogey has taken place.