Pete...have you looked at the summary and route plan type, you have a chocie of 5 routes, fastest, shortest, recommended, avoiding tolls, and prefering motorways. I myself am going to Torreleo de Montgri and have planned my route with this, first I thought the shortest which took me along the A20 via Tours and Limoges but having looked its I'm going on the A75 (E11). The route its probably given you is avoiding Paris. Have a look at, from Calais,
A26 and A1 toward Paris, then you hardest part will be Paris itself, I've done this no problem but you need to be alert and stay under the signs your heading for, you will need Bordeaux and the A10, follow that and at Orleans go on the A71. You should now be on the E11 although I never bother with the E parts I follow the French motorway numbers. You will need the A71 towards Clermont Farrand, it then becomes the A75. You will cross the new Millau viaduct and shortly after might have a bit of normal road but then you go on the A9 and on into Spain. Check this route out, I've tried a couple of sites and they all recommend this as the quickest, but no the shortest. Its 95% motorway maybe more. I could have saved myself typing this as I've just checked and via michelin give this as the quickest route. Hope this helps
Mike