In Spain one has to have, as part of a car insurance certificate, a 'bail bond'. That is to say that any driver involved in any accident where there is damage to a third party or injury to a person, is arrested until the case is investigated by an examining magistrate. Unless one has a 'bail bond' as a part of one's insurance, or a large cash bond is put up, the party - apparently innocent or not - stays in jail until the enquiry is over. Most modern UK insurance is valid for European Union countries, so-called 'green cards' are becoming less automatic than they once were. Fortunately Cornelius had one. Even more fortunately it listed Spain as a valid country and therefore included the bail bond.
When he eventually left the police station at Medina Sidonia, Cornelius was minus the car. It was undrivable anyway. Moreover, he couldn't hire another because he had left behind all his documentation, including his driver's licence, impounded by the police. It wasn't that officialdom was picking on him, it is simply how things work in Spain.
Cornelius himself, however, was relatively free. It had been necessary to give a local address, so he had booked by phone into a hotel in the town, but he did not have to stay there. After taking all the luggage to the place he hired a taxi and had it drive him to twenty or so miles to the hospital to consult Ian. He could hire a car.