Forget it.  I have had two separate quotes, one from a guy advertising on this site - who did me the courtesy of a very long and thorough reply explaining where the money goes. and another from a trailer fabricator in Amposta...GAD or something similar.  anyway, you're looking at 3500 euros and probably plus IVA.  There's two different surveyors and loads of people to pay, lots of pieces of paper and a load of old Spanish burockracy.  The 1500 kg boat trailer I wanted to legalise could have been bought new from BCN for 2800 euros.   They do not recognise any other country is capable of building roadworthy trailers and only theirs are good enough.... oh yes,  of course...domestic market protectionism again.  The Third World continues to flourish here.