Damp

Tenancy Deposits

My tenants are due to leave at the end of July. When they moved in July last year, the bathroom had been freshly decorated and I verbally advised to keep the small window open while having a shower/bath to prevent damp. Due to large commitments last year I wasn't able to check the flat thoroughly though I did visit every 4 months. Consequently, 2 months ago I discovered the bathroom walls and ceiling covered in damp and promptly had an extractor fan fitted. I also had a professional have a look. He concluded that it's surface dampness though it does need a thorough clean and re-paint, it would have been prevented with the window open or a fan. I lived there for 6 years and never had a problem as I kept the area well ventilated. Where do I stand on this matter? Do I ask them to clean it up and re-paint it at my expense, which I wouldn't mind, do I charge them for cleaning it up and pay for the re-paint or do I keep the money from the deposit for the whole job?
Although it's not so bad in the kitchen, there is a slight problem in one corner so I consequently wrote into the contract to keep that window open. I know this wasn't adhered to as I drive past often and so told them a few times. Again, do I charge them for the clean up and re-paint of this damp?