We got a dehumidifier from LittlewoodsDiirect for about £99 during the middle of winter because we had a damp patch pop up right next to DD's cot. So we did what you did, sprayed it with bleach and tried to keep the windows open, but with a small child windows can't be open all the time. Then a patch appeared on the ceiling and spread halfway across the room, so we did the same...bleached it and then bought the dehumidifier...we've had no damp problems since. It took about a week to get all the moisture out of the walls so it ran constantly during the daytime, now I just make sure it runs when I have laundry to dry. It raised our quarterly electric bill by about £35.
It is in our lease that we have to keep the residence free of damp so we couldn't get reimbursed for the cost, but best thing to do is check your lease and if its not specified you could ask for reimbursement.