THM, I can't recommend a gestoria, but I have been through the whole process with a UK registered motorcycle.
It is extremely slow and time consuming, so while you search for a gestoria I would recommend taking the first step yourself.
You will need to get an ITV for the vehicle, not sure how you will do this if it has been impounded mind you. Before you can get an ITV, you need to get the cars specifications from the manufacturer, the Spanish call it a ficha tecnica. So I would contact the manufacturer in Spain, as request one. A friend who brought in a Mercedes had quite a wait, in my case Yamaha needed a month. With this you can then try and get an ITV, and once you have that the whole admin process of paying import duties etc can start, which is where the gestoria will be useful.
Aren't you glad the EU single market has simplified these things so much !