Hi rolley and welcome to the site.
Firstly, the Spanish now call it CO2 emission tax instead of import duty. It is worked out on the value of the car, the level of CO2 emissions and a rebate given for each year. It was posted on here a while back with the actual make of car, the year of manufacture and the amount to be paid.
However, you can keep the car on UK plates for up to 6 months provided the MOT and road tax is up to date. If you are coming to live, you can register on the padron (equivalent to the electoral roll) and, if you register the car on Spanish plates within 60 days, there is no "import" duty to pay. Whilst on UK plates, only a British MOT and tax disc is acceptable.
PS Signals 29 years but left a long time ago.