I heard from my friend in Santander and this was his message:-
"Sorry for not having written to you before on regard to your question about where your acquaintances can leave their bicycle´s boxes on their trip to Spain.
I´ve asked the airport, FEVE and the Coach Station and none of them have a place for this purpose. In FEVE, they have a kind of luggage place but not for big things neither for keeping them for so long (15 days I said), they said that maybe RENFE has it but they doubt it. I went there anyway, it was lunch time and nobody could give me any information, so I´ll go next Thursday and will tell you something afterwards."
FEVE is the Railway station,
General cycling advice ( NOT technical ! )