I'm an 83 yr old frail woman in a small blok of flats with fire doors.
The outer door is now so heavy for me that I can't get out of the building.
Can you clarify the duties of the landlord for me please?

The common areas of blocks of flats are subject to the The Regulatory Reform (Fire Safety) Order 2005 and require an adequate means of escape. If you cannot get out of the building then the landlord is failing in his/her duties and if they is not prepared to rectify it, then I would suggest you go to the local Fire and Rescue Service who are the enforcing authority. http://www.firesafe.org.uk/uk-fire-rescue-services-details/