The Lease is the document which governs the relationship between the Leaseholder (flatowner), the building owner (Caledonian Roofdwellers) and the Management Company.
It defines the extent of your flat (which is self-contained) and defines the Common Parts of the building which are the responsibility of the Management Company.
A generic copy of the lease (all are basically identical) is stored in the Google Drive to which you have access. Other pages in the website describe some of the detailed provisions of the lease.