Ram has the right legislation. In effect you have a legal duty to any user of the building for whatever reason.
You are therefore having to justify what you did in the event of a claim. Since 2003, despite the Daily Mail, if a burglar is convicted then they have to ask the court for permission to claim.
As a rule you have to consider whether someone might come in contact with it so with no access to the roof, over 2.4 metres and signage will pass the test.
If you plan to use the roof for recreation or a fire escape then its inappropriate. If that window is vulnerable you might consider a security shutter or bars or reinforcement.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com