One of the bus companies that stops in Santillana is La Cantabrica. You can contact them at the link below for schedules. If you are based out of places like Santander, Comillas, or San Vicente de la Barquera, the bus is a great option to visit this town.
You can request a schedule between the cities you'd like to visit by email by writing to:
Bus from Torrelavega
Couldn't find the bus station to get from Torrelavega to Santillana Del Mar, but know it's outside the FEVE station now. only cost one euro there and the same back! got a taxi there to start... only cost 8 Euros!! The bus coming back is on the main road opposite the coach park! But why go to Santillana anyway!