Answer to the first question is yes a Structural Engineer is the person you will have to contact to advise you on what remedial work is required.
The second question depends on your mortgage provider.
For instance they will not allow their surveyor to class the Loft Conversion as a habitable room and will ask for a Completion Certificate if the conversion was to add any value to the house.
Thirdly if you did want to convert the loft properly then a Loft Company will check to see if there are the right beams (RSJ's) in place and also check things like floor joist specs and insulation among a number of other things.
I hope these answers are helpful and wish you all the best and hope the outcome of this matter is favorable to you.
