INAL but I assume that the tenants of these units have to sign a contract, which stipulates what happens in the event on non payment. It seems likely this would say that any item left in the unit would be used to recover any debts. However, in exercising such a clause it would be up to the creditor (i.e you) to ensure that any item does indeed belong to the debtor.
However, no contract can contravene other legal rights. _Chris_ is right to raise the point of courts. It would seem right to go to small claims court (assuming that the amount is less than £3000, as you're in Scotland) to get proper authorisation. But maybe it's more?
Once you have the appropriate documentation in place, it should be possible to get the V5C from the DVLA, etc. There's companies that specialise in selling cars in these sorts of situations, like this one. Maybe they'll take the cars in exchange for this Eurofighter Typhoon model thing.
You could even try inviting this guy: