Hi Roy, thanks for your posting, I have slightly edited my previous posting.
The situation is that a UK car can't be used for more than 6 months in Spain. So it could be here for longer than 6 months (e.g. garaged) as long as the use doesn't exceed the 6 months.
Personally, as I'll be in Spain less than 6 months in a year the car use will fall within that 6 month period. Several different people driving the same car would have to carerful.
Paul, on the MOT I'll be driving back to the UK for a service and annual MOT.
The British Embassy says:-
"UK nationals who are not residents may bring their car into Spain, but
neither they nor anyone else can use it in Spain for more than six
months in any one year."
Here's a link to the Embassy web-site:-
http://ukinspain.fco.gov.uk/en/help-for-british-nationals/living-in-spain/cars
Ian