Home Ownership contacted the Capital Works which were carried out the works on the Curnock Estate and they spoke with the programme manager for Camden Town. After that conversation they informed us that:

Camden Council actually owns the windows but not the front entrance doors to leasehold flats. The leasehold agreement demises the front entrance doors to leaseholders.

They also informed us that this would explain why the repairs team were unwilling to look at the letter box issues of the leaseholders, as any fixture to the front entrance door would belong the leaseholder, and they would have its repairing responsibility.