Job description

I have just moved into a ground floor Victorian conversion flat. My homebuyer's survey flagged up potential damp below the bedroom window. As a floorboard was loose I took up and have found that at least one joist is rotten and just crumbles on touch. I need someone to check the extent of the damp in this room (3.5x2.8m), apply damp proof course or other work as necessary and to replace all rotten joists. I believe floor boards are good. Also, where the fireplace hearth used to be the floor is lower as just concrete and I would also like this levelled at the same time if possible (concrete?). I will be removing the old carpet and underlay and would consider lifting the floorboards myself. The work needs to be done before redecoration and furniture is moved in to the room. Thanks