I have just returned from the Guardia Civil
The first officer I spoke to agreed with the info I was given at the National police. However, when I persisted, he called Trafico and let me speak to them.
They said anyone with the appropriate DL, including a visitor to Spain, could drive a Spanish Reg vehicle with the owner's permission.
On the point of the written permission,
They said it was not obligatory but a sensible thing to do, as a police officer stopping a vehicle being driven by someone other than the owner might have difficulty establishing that person had not in fact stolen the vehicle, and if in doubt might detain the driver until the owner was contacted.
This message was last edited by johnzx on 18/03/2015.