The toddler was filmed by a neighbour who alerted police after spotting him playing on the ledge, seemingly oblivious to the danger.
Debbie Green held her breath as she watched the boy wander along the edge, swinging his legs out over the sheer drop.
She said: "My heart was in my mouth. I was sure he was going to fall to his death.
"I grabbed the phone to call police and my husband got his camera with a zoom to see close up that it actually was a little kid and not some doll.
"The police arrived after about six or seven minutes but it felt like hours. I've never felt so scared, it was horrendous."
Debbie's husband John, 42, said: "I feared I was about to film this child's death. I was amazed he was able to keep his balance."
Police who went to the flat on the eighth storey of a tower block say they found the boy's mum and a man asleep inside.
Officers said she claimed her lad had
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