Her remains were finally found when bailiffs visited the property in the 1980s-built block to collect unpaid debts.
After getting no answer at the door they got into the property through one of the open windows and made the grim discovery.
Shocked neighbours have spoken of feeling guilty that it had taken such a long period of time for anyone to realise Mrs Leitrim was dead.
They described her as being friendly and softly spoken with a Scottish accent.
John Stanley, 55, who has lived in the building in the Throop area of Bournemouth for the eight years, said: "Everyone is so shocked.
"We didn't think anyone was living in the flat. Her car went and we just thought she had moved out.
"There are cobwebs in the windows and we used to joke there could be a body inside but we had no idea.
"When the bailiffs and locksmiths came to the flat they entered the property through two windows that had been left ajar.
"The windows were slightly open and they used a coat hanger to get inside.
"The locksmith went in and said 'I think there's a body'. It was so decomposed he couldn't tell."
Ruth Evans, 59, who has lived in the building for six years, added: "It makes me feel guilty. You think of all the times we were out in the garden and she was just lying there.
"When her car disappeared we all thought she had moved away. When I heard there was a body inside I couldn't believe it."
Debbie Small, who has lived in the same street for two years, said: "It is really sad. It is a really friendly street and we all know each other. I can't believe it."
Neighbour Mr Stanley said: "Bailiffs arrived on Tuesday because she had been defaulting on her mortgage payments since May last year.
"It turned out she had been making automatic payments from her bank account for four years after she died.
"To think she has been lying there all this time is heartbreaking.
"She was lying on the floor next to the bed with her hand over her chest. They think she probably had a heart attack."
A spokesman for Bournemouth coroner's office said formal identification of the body had not taken place.
They said a post mortem examination will be attempted but it is highly unlikely to identify a cause of death.
It is thought they have been in contact with Mrs Leitrim's next of kin.
A spokesman from Dorset Police said the death is not being treated as suspicious.
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- Daily Mail