Hi
I am no lawyer but as I understand it you are entitled to with hold your rent to a landlord if repairs that need to be carried out are not being done. However if the matter goes to court then you have to prove that you have that money to pay the landlord on completion of the repairs. Just inform your landlord that it is your intention to with hold rent until such time as repairs have been carried out to a satisfactory standard. Also you can contact the enviroment agency on the matter of the mould and if need be they will intervine and serve orders against him
If the month is up and you still have that months rent then that is your deposit back.
Should the landlord not pay you your deposit back you are entitled to take him to court for the amount you feel he owes you. You will need to have evidence of what your room and accomodation was like when you moved out of the property and that no damage was caused by you in the time that you were there. Maybe handy to take pics now and then on the day of the move to prove that it is in the same conditon. In the same respect the landlord also has to prove that you caused
Hope this helps