My understanding is this: Camden separates out work as both “estates” (work affecting the entire area) and “blocks” (works just affecting your building). Pavement or central boiler works are therefore estate costs as they benefit everyone, whereas the concrete repair works that we have recently had carried out here will be charged on a “per block” basis. For another example, lifts are always charged to a whole block regardless of whether the ground floor uses it or not (and this methodology is often specified in the lease).
At a guess you need to ask Camden how they define your estate and whether they have done this so consistently over the years? It could be that at some points your building has been subsidised by the other building, and at other times vice versa. Often Camden, I suspect, would claim that treating both buildings together makes for more efficient management (and therefore costs).
To further understand the methodology behind the breakdown do contact Leaseholder Services and they should actually get back to you with an answer.
(Leaseholders’ Forum member)