In the Daily Mail last week there was comparisons of fuel worldwide and it said that it was about £1.15 in Spain for diesel and £1.19 in France, don't know how accurate this is though. Going to Spain in August, can't wait!
This is a very useful website to find the cheaper fuel stations near the place you are staying. Unfortunately, the site is only in spanish.
If you are looking for a more general information, you can browse the monthly prices reports that are available in another page of the same website (http://www.mityc.es/Petroleo/Seccion/Precios/Informes/InformesMensuales/2008/). Open any of the pdf files and go to the last page: there you'll find mean prices of fuel in the whole country for every month, taxes included.
Prices in the last week of may were €1,1933 / ltr (unleaded), €1,2395 / ltr (diésel). Anyway, regional differences in the fuel prices are quite huge.
Hope this helps.
Don't forget to leave a review of the French and other European campsites you have visited!
Thanks for the info. We were just trying to find out if prices were cheaper or not than here in the UK. It looks like it could be a bit cheaper which is nice.