If it's a purpose built flat block then the correct guidance is the LGA Guide to Fire Safety in Purpose Built Flats and is sympathetic to existing premises. Your existing doors in such a small block, if notional fire doors compliant to the standard of the time, may suffice, although upgrading is always useful
http://www.local.gov.uk/c/document_library/get_file?uuid=1138bf70-2e50-400c-bf81-9a3c4dbd6575