OK .This may not be the definitive answer, but here is the law as I understand it.

You are allowed to have a foreign registered car in Spain for 6 months, after that time you should have it changed to Spanish plates, which I believe can cost up to a thousand Euro or more.

For that reason many people do not bother, and although I am sure that nobody would advocate that you break any laws, the likelihood of being charged with any offence is remote. The police over here do carry out spot checks but these are usually on commercial vehicles to catch illegal non-EU citizens, or to catch drink drivers. In my experience they simply cannot be bothered with finding out exactly how long you may have been in the country (bearing in mind that your passport would have no entry stamp and you might tell them a 'porky').

If the police wanted to be 'stroppy' they have ample opportunity to do so, even on correctly registered cars. For instance, a driver over here must carry a whole bunch of things in his car at all times, including:-

All documents relating to the car.
Passport and driving license (original copies only)
2 warning triangles
Enough florescent jackets for the driver and all passengers
Spare bulbs
Fire extinguisher
First aid kit
Spare glasses (if you wear them for driving)
The list is probably longer but that is all I can think of right now!

As you can imagine, there are very few (if any) drivers over here that are completely legal!

There are plenty of insurers over here that would cover your car when your initial 3 months UK cover runs out.

Thanks for the reply Oly Daze.

I was in Spain for 3 months at the beginning of the year and built up my "Car Kit" so that's not a problem.

Having a foreign registered car in Spain for 6 months, is that in any 12 month period or could I come home then return and start again?

During the 3 months I was in Spain there were spot checks every couple of days, mostly on roundabouts, and as you say I don't think they could be bothered with the hassle. They just waved me through each time.

Having been in Spain for 3 months I now have the urge to try longer before finally committing myself to a permanent move. After which I would sell my car and buy a LHD. The general opinion seems to be it's cheaper to buy than register with Spanish plates.

Again, this is only my understanding of the laws.

I believe that the rule is that you should register your car on Spanish plates if you have been in the country continuously for 6 months. I imagine that anyone asked by Plod would probably claim that they go back and forth to the UK regularly, only staying in Spain for a maximum of 6 months of the year and that they find it cheaper to insure in Spain and drive in the UK on that insurance.

It is only my personal experience, even though I have a Spanish car I have never once been stopped by the police. British people that I have spoken to seem to think that the police cannot be bothered to stop them in UK registered vehicles, perhaps because of a potential language barrier.

Finally, the value of the car would probably determine if you would want to register it or not.