Bunter wrote on Mar 28^th, 2014 at 4:11am:

Law passed 20th March 2014 Vehicles on British plates used in private or business must be transferred to Spanish plates after one month.remember the word is MUST. The only exceptions are people on holiday. Police have various ways to check British vehicles ie insurance/MOT/and Reg/Local docs Both about you and your vehicle. This is going to be enforced.Mossi blackberrys have direct access to the police/insurance data bases. In short all UK vehicles after one month must be on Spanish plates.Large fines as well as vehicles being impounded await.The days of buying an old banger off ebay and if impounded who cares are gone they will now take you before a court and get the cash for the fines. I am sure your bar room lawyer will tell you in Catalonia they are years behind it will not happen. Remember the Mossis know all the back roads/tracks. Every day you can see them doing their little road blocks and they will be checking UK vehicles and the one's I have encountered speak English.

So what happens in my case? I have a pickup truck which is fully MOT'd taxed and insured in Gibraltar with a uk company and the insurance is for any amount of time in any European country, with breakdown cover.
Being a pickup it is classed as a light commercial vehicle, (even though I only use it for personal use, not business) so therefore it cannot be put on Spanish plates. Their law from last year.
I also spend time in France and the UK so the vehicle is not permanently in Spain, but is off and on throughout the year.