People living and working in Penywern Road say the fire happened in a flat occupied by the elderly woman.
The blaze occurred on March 13 , with London Fire Brigade revealing three days later that the woman treated for smoke inhalation had died.
One couple, who did not wish to be identified, lived in the block of flats and were evacuated.
They said: “We woke up smelling smoke and thought it was someone's toast burning. But then it started smelling quite strong.
"We looked out of our window and saw two girls outside telling us to get out and that there was a fire in the building.
“We left the building, and were outside for about an hour and a half. I think the fire brigade were trying to find access to enter the basement because there was an old lady inside.
"People were saying she was unable and was stuck inside the basement.”
Another witness, who worked at a hotel opposite said: “I was told by other people there was an old lady in the basement, but I didn't know she had died. That's very sad.
“There was a crowd on the street and people were asking what had happened.”
Fire crews believed the fire was caused by flammable items left near an electric heater.