You need to notify the owner of the car in writing that it has incurred and is incurring storage costs.
Also that there is an outstanding debt for the cost of repairs.
Give an up to date total.
And give notice of your intention to dispose of the car to recoup some or all of the costs, any residue will be forwarded to him/her.
Set a time limit, eg. 14 days.

Wait for his / her reply and at that time ask for the V5.
If no reply give notice of your intention to apply for a duplicate V5 and to then sell the "Car".

You get into trouble when you don't give notice of your intentions.

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