A bit late with a reply but if you're arriving in Santander at 5.30 there's a handy site just six to ten miles from the dockside. It's Camping Virgen del Mar which is over on the north side of the headland. You can approach it through the town which is the 6 mile ride or round by the motorway. The GPS coordinates are N43.47540//W3.87476. The site covers a lot of ground but in the winter they only have a small portion of it open, so when finding it, you need to drive past the site, down to the carpark, and then take the road on the otherside of the site.