we were pulled by the police in 2007 in Spain.

the Police were going to fine us for not having 2 mirrors.. no GB sticker on the bike. and a dirty number plate { no use having a GB sticker on the panniers than taking them off and leaving them at the camp site }
one American in our group had his loaned bike towed to the police station because he only had a US licence .

you also have to carry your licence ,insurance and V5 with you .

The police are on the ball in Spain, as they even cleaned our tax disc to check they were in Date !!!