Hi Nick and all
Our internet is a little too slow to look for the source of the Duero from googlemaps, but I have been there several times. There are various ways of getting there, but the quickest way is to drive to the stunning Laguna Negra which is a must if in Soria and walk to the Pico de Urbion (2229m), from there follow the GR86 in the direction Castroviejo and you pass the Duero source on the way. It beautiful countryside with several glacial lakes throughout, and as Clive pointed out some centenarian Scots Pines.
Ive added a map (hope its a clear image), the coordinates on the map are : 510 x 4651, mas o menos !
Let me know if you need any further info or if you want me to email the map.
Cheers
alfredo