It sounds like there may be a historic right of access from one part of the estate to another but this may now be in doubt given subsequent development. The reason for this is because a right of access is strictly construed and can't be unduly onerous if there is for example a change in the nature of the land or ownership. The classic example is where there was a right of access for one landowner and then the land was divided into housing units. The one right of access can't then apply to several units as that would result in a greater burden than before. However, can you explain your particular situation a little more and I will try to help you.
If the access was originally only to one flat in the building then arguably if it was converted to one building then the access would no longer be needed. As a matter of law it will still be on the title, however, and you may need an application to the Lands Tribunal to have the servitude extinguished if it is no longer relevant. I would need to see the title deeds to give a fuller opinion. You can see more about the Lands Tribunal at their website.