As far as I understand it you don't have to have a spanish licence. You can keep your UK(it's an EU Licence anyway) licence if you prefer but it must be registered with the Spanish equivalent of the DVLA. Also if you are on a UK licence you still need to have a medical certificate just as you do with the Spanish licences. I was arranging my sons motorbike licence recently and enquired about the cost of changing my licence and was told it would be about 50 euros.
And finally the Agentes Rurals were in the area recently checking out abandoned(none working) vehicles on fincas. It seems it is an offence to keep these on your land.
Bruixot