Brian,
We booked our stay in Santander through booking.com, excellent booking service and no commission or fees paid to them. Hostal Cado Mayor was the original hostal but because it was full they arranged a nearby hostal for the same price. This hostal is within a few minutes walk from the bus station, in the centre of the town. Our hostal had a resident owner and he was open to late in the night as he lives there.
For your further information Santander is a late night place, dinner starts at 8pm or later. 10 pm arrival probably is not strange for them. The street which Hostal Cado Mayor is situated (Cadiz) is riddled with cheap places to stay. Check with the hostal directly or better still contact booking.com when you request the booking to ensure the hostal will open the door for you if they do not have a 24 hour reception.
Although called a hostal the place we stayed had a private double room, TV, ensuite bathroom, telephone, internet connection, all for 40E for the room.
http://www.hostelworld.com is another place to book a room but booking.com has full time office and they will get in touch with you instantly by email or by phone or fax.