Ok, lets start by saying that most of the time a dry house is a happy house. In my experience 95% of the problems in house are due to it being damp or wet. Can you get access to that part of the roof? that needs looking at as its clear there is no felt under your tiles. It does look like this might have been going on for some time. You need to check under the roof to see if the water has started to rot the sark boards and batterns that are holding the rest of the roof up. If this is the case they might need cutting out and replacing.
To address what is going on inside the house you need to fix the roof first. That Mould isnt going to go away either. I would suggest given the state of it that the plaster is removed back to brick and once its all dried our and the roof leak is fixed you can plaster board it and get it skimmed over and it should be all good again.