Now I know nothing about metal detecting .. but a good friend of mine is very keen, but got "sent home" in August under a rather dark cloud having been arrested ... we all asked about it .. and I have a few bits left on the computor .. apparently it's an odd situation .... the rules on "treasure" in spain make metal detecting almost illegal .. but there are "ways" ... apparently 
http://www.thespainforum.com/f271/metal-detecting-spain-259958
also
http://www.torrevieja.co.uk/forum/viewtopic.php?f=13&p=547753
from another source :
I found in "El Pais" one of the principal newspapers in Spain. That the cause for that law was because of some people stealing important archeological artifacts to sell them on the black market. Here is the translated article.
The Law of Patrimony will prohibit the metal detectors
The new norm anticipates an official body of inspectors
P.E. - Cadiz - 29/10/2007
The new Law of Historical Patrimony of Andalusia, that is anticipated to approve in the autonomic Parliament next the 14 of November, will harden the measures against the exporadores and those that damage the archaeological rest. The norm anticipates the creation of an official body of inspectors who guard by the security of the deposits and elevates the maximum fines of the 600,000 present euros until the million euros.
One of the measures more showy than will include this renewed legislation will be the express prohibition to use apparatuses to locate pieces of the past. The metal detectors will only be able to be used in zones determined, during a concrete period and with express authorization of the administration. The Meeting wants to thus end the traditional image of the metal finders raking the ground of Andalusia.
For the provincial archaeologists this measurement supposes a firm step ahead to protect the patrimony of the community. “It is a practice very extended and very dangerous because the custom exists to take of unpunished form elements that can be very valuable ", it explains Ana Troya.
The Civil Guard indicates that the carriers of these detectors are, mainly, retired or people without fixed work who dedicate themselves to look for in archaeological deposits, the beach or the field to rob pieces and to take them to houses or to sell them in the black market. “Not always they obtain great amounts of money but they cause a great damage because they eliminate the context of the deposits ", it details the archaeologist. The new law will harden the sanctions to the carriers of these apparatuses. “The one that remains with a currency is being with part of all we ", maintains to its companion Bulging Alfonso.
In the last three years the volume of information has been tripled that leave the Delegation of Culture on possible archaeological affections. Even so the destructions and expolios are continued producing. “We tried to arrive as rapidly as possible although there are times that are too much behind schedule ", it laments Ana Troy. The archaeologists of the Meeting have an anticipated car to this type of performances. A vehicle prepared for the sentries of the past gaditano.
HTH 
Thanks for this. I haven't got a metal detector or ever done any but I just know that there must be tons of treasure here like there is in Britain as the Romans were all over Iberia. Likewise I can understand the official line as Spain isn't very well advanced in "developing" its historical sites. Only a mile from our house there is a copper age (4000 year ago) dolmen site with 9 (so far) chambers. It was fenced recently but is still free and open to go around. A neighbour showed me a perfect stone tool he found which I suspect was a ceremonial one. As I expressed an interest he gave me a carrier bag full of similar everyday fragmented tools he has collected over the last 30 years.
It would be no use asking the Guardia Civil. Mostly they turn a blind eye if people keep their heads down but jump on you if you are flagrant. Even though all land here (unless fenced) you are allowed to walk on - metal detecting, unless it has moved on, is a highly visible activity. there is always someone around in the countryside here!
As far as maps are concerned there are quite good detailed maps available here but most folk don't know of their existence. They are based on old army maps. That is where I found out about the old Roman Road. No one in our local bar knew about it and were fascinated to actually see a map.