If you mean obtain the right to sell the vehicle and recover your costs then you need to serve the Torts Act on the owner ( I think that's how you spell it ). Talk to your local small claims court, that's where I got my info.
If you mean disposing of for scrap, with their permission, then it's just a call to the local scrap merchant ( if they'll take it ! ).
If you can't contact them and therefore don't have their permission to scrap it then talk to the DVLA.