Gap between Granite wall and Plasterboard

[teddy_amok]

Hello, Me and my wife live on the 2nd floor of a 2 storey building. Below us is a property leasin company. We have all of the 2nd floor with access to the loft. The builing used to be as one but the 2nd floor was converted. The loft is poorly insulated but I am about to fix this.

The building is solid granite stone exterior with a 3-4 inch gap and then plasterboard internal wall.
Since we have lived here we have heard random noises in the walls at both ends of the flat. It just sounded like plasterboard falling off from the inside of the wall.

We have really bad condensation issuse during the winter (we use copious amounts of small condensation traps etc) which I am going to address by dry lining the place. Wont be easy though.
I will start by draughtproofing the place (plug uncontroled air gaps,properly insulate wooden bays aroubnd double glazed windows) and then insulating loft (cold loft insulation for light storage. very old building.100+ years and so slates have the expected gaps that keep out rain but also let loft breath).

The thing which troubles me is, when I was in loft last week and I went to either end of the loft (not front and back, they seem sealed) there was a gap between the external wall and the plasterboard, meaning I could loook straight down the gap between wall from the loft.
At first I though this is how it should be but now I have the question, if i insulate the loft, will the cold air coming from the roof go down the gap between the external wall and plasterboard which in turn will male plasterboard very cold, which in turn might be adding to condensation problem.
Should I seal this gap at the top in the loft or will this create further condensation in this soecific gap.
I can physically see a gap from loft in frot or back of building.

Thanks so much for reading my post. Really appreciate any help.

Kind regards,
Ted