Hostal Cruz Sol sometimes has vacancies even if their website shows they don't. I've emailed them and got a room even when they look like they haven't.
Ask that you stay in Hostal Cruz Sol itself, not it's sister Hostal below, as the airconditioning in the forementioned is waaaay better.
I've stayed in Hostal Martin too, but prefer Cruz Sol because of it's position, just outside Plaza Mayor and the fact that every room, as far as I know, looks outwards, which they don't in Hostal Martin.
Even though you give Cruz Sol your credit card details, I think you can cancel penalty free almost up to arrival.
Wish I was going again. It's great in September
Jo