I have been in touch with Caravan Club and others since the previous postings. The situation is still not fully resolved concerning the dimensions of the plates, although the dimensions posted earlier are probably OK. What is clear is that the rules apply to vehicles or combinations of vehicles (e.g. car + caravan) over 12m. overall length, and that the plates have to be "homolugated" in Spain - i.e. marked with a Spanish approval code.
Enforcement of the rules seems very variable, sometimes down to the discretion of the individual officer, and may well vary from area to area. This is not very helpful to us, but personally I would find it a very uncomfortable feeling to know I was travelling some 2000 miles through a country while breaking their law, however unreasonable it may seem. Not the way to have a relaxed and enjoyable journey.
So far as supply is concerned CC have suggested one supplier in Spain but have yet to publish the link - I suspect they may be being super-careful after the "false start" with this matter and self-adhesive plates. Also, they are understandably cautious about anything which may appear to be a recommendation of a supplier with whom they only have tenuous contact.
My only suggestion is to stop just in France (e.g. St Jean de Luz near Irun if using the west coast route) and then go across the border solo, buy the plates and return to France and fit them. With the large volume of HGV traffic at the border areas, I guess that there will be ready sources of supply - try the garages that handle HGVs etc. If not in stock they seem to be able to get them very quickly - in a couple of hours - and they are often open until 7.0pm or later.
If you look at www.caravantalk.co.uk and their overseas touring topic you will find copies of correspondence from CC to some members and the links to possible suppliers. If you look earlier on this topic you will find a posting from Lutz which gives a llink to a .pdf attachment which shows the dimensions and requirements and also indicates that the law dates from 23rd December 1998. If your Spanish is reasonable you can look at the spanish Directorate of Traffic (DGT) site - all 93 pages of requirements for plates - and see the whole can of worms.
When checking the length of your outfit, don't forget to add the towbar to the "book" figure for the car's length and then to subtract about 55mm from this to allow for the overlap between car and van when hitched up.
Finally, i suggest you arrange for the platesto be readily removable as there are some questions concerning their legality in other countries, and they do little to enhance the rear appearance of the van.