Yeah you shouldnt tax it with just that. I had this problem once so I phoned the DVLA. If the prev owner has not sent the rest of the v5 then you need to go down to post office and request another form with proof of insure (ive forgotten the name of the form you need). You can then get a tax disc at the post office, then just send the form off to the DVLA and they will send you a new v5