Lillylou
I’ve watched this thread with some interest.
To me it seems that in the dim dark past some deal has been done with a former owner of your lot.
Before you proceed any further with your plans, I would find the meeting minutes where the exclusive use was granted.
I would then use this to formalise your exclusive use by getting a by law granted. The bylaw is the only means you have to enforce your rights.
For example, without a bylaw (which I understand you do not have now) any other lot owner would be able to enter the roof area and you would not be able to prevent them.
The bylaw giving you exclusive use entitles you to claim an ” ownership” of the roof area and prevent others trespassing on it.
You also need to be aware that exclusive use arrangements often come with a requirement to pay a rent to the OC. You are getting an ongoing benefit, but the OC only ever got one payment ( if it actually was made).
Your development almost certainly requires council approval. Since you do not own the roof space, it will be the OC making the application. Therefore it’s not just a case of getting a special resolution passed (75% passing the motion), you may need a unanimous resolution
I think you have a difficult road to travel.
Robert