In reply to JimR:
Hey Jim, my sister is an English teacher operating just south of Santander and knows the region pretty well, but isn't a climber. I've been trying to get her to put up some stuff about climbing as her Spanish brother-in-laws are pretty keen and have some local knowledge. Check out her blog for some general tourist advice, and she may be able to get you some more info if you contact her through the blog(also on Facebook).
http://ratoncantabria.wordpress.com/
Have done bits of sport around where she lives in Reinosa, they seem to have little crags all over, but as Alun said, some of the best stuff is in the Picos, particularly naranjes bulnes (can't remember spelling)
which has been something I've wanted to get on for a while. Only 1hr 30 ish from Santander to the cable car up into the mountains near Potes, which is an easy walk to some of the big stuff.
If you do head that way there is a guy just on the north edge of Potes who runs a climbing shop with a small bouldering wall in who's got all the local knowledge (including loads of pencil drawn topos in binders), and speaks really good English. He's told us some of the roadside crags when we'd been there in winter, and hired via ferrata gear pretty cheap too. Might be able to dig out his number if you're interested.
Also the drive to Potes is up a pretty cool gorge, with some interesting rock obstacles, almost worth it on it's own.