POsted in the wrong place before. Our transit burnt out at the side of the road and was written off - how do we claim the tax back being as the tax disc was somewhere in a melted pool of plastic??

Hello everyone , hope you can help as noone else seems able to. Recently we were unlucky enough for our Transit to decide to spontaneously combust at the side of the road. It has been written off and turned into petfood cans - how do we claim back the unused prtion of the road tax as the tax disc is no more??