Be carefull though arriba...they will put miniature warning signs up one day and you will come back with some heavyweight inbred demanding 200 quid to unclamp your motor...be warned.
I think I'm right by saying that clamping posters/information have to be a certain size (pretty large), and the signs have to be a certain height off the ground so all can read, the happy clampers must also hold and display there security licence (this has cut out a lot of firms using this system). There must also be information throughout the area telling you where to go and read about the clamping laws around the area.