If you read your documents of purchase carefully, you will find that the management company are likey to be responsible for the entire external envelope of your duplex (external walls, etc.). As mf1 said, make sure you actually own the attic space, which is different from 'having the enjoyment of'.
If you can make a case that you are adding value by carrying out the works, it will help.
In terms of the works themselves, you are likely to need both planning permission and a fire safety certificate for the development. One of your issues in this regard is safe means of escape from the attic to the exterior of the building, which might involve considerable works to the remainder of the apartment