To sum up, becoming the registered keeper on the V5 does NOT mean you are the legal owner.
When you buy a vehicle on SORN (or not) any outstand fines do not transfer to the new keeper. But make sure you SORN it as soon as you get the logbook through in the post.
You should write to the DVLA for the keepers last known address, write a letter, take photos and copys, send it recorded. You then have proof you have made reasonable effort to contact the owner. If no response within a reasonable amount of time, you have the right to claim it as yours. Register the vehicle in your name, then do as you see fit with it.
(typed slowly at the same time as pat_pending) ^