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Margaret Bradshaw was born in the UK, but lived and worked elsewhere for most of her adult life, so simply assumed that she wasn't eligible for a state pension when she came back to the country in 1990.
However, the great-grandmother qualified for the state pension after her 80th birthday back in 2001, meaning that she has missed out on 20 years' worth of money.
Her daughter was the one who revealed the mistake after reading an article about older people not claiming their pension.
That led her to investigate whether her mum was able to get the money, eventually discovering that she could have been claiming £82.45 per week since her 80th birthday.
Now, after seeking help from Sir Steve Webb - an expert in pensions - Margaret started claiming her pension two weeks ago.
The former nanny and hotel worker, who is now suffering from dementia and lives in a care home, has also been awarded backdated payments of £4,000, but she'll never get the £75,000 she missed out on.
Daughter Helen, 78, said: "I read this article about how thousands of people over 80 aren't claiming the pension, and it made me question if my mother should have been getting it.
"I had never even heard of an over 80 pension until a few weeks ago - we were never made aware of it when mother turned 80."
"I had been getting quite nervous about mother's financial situation for some time as care homes are very expensive, so I felt some relief learning she was entitled to more - even if she missed out for 20 years.
"But there might be hundreds of other people out there who don't realise what they're entitled to."
Margaret was born in Croydon in 1921, but spent nearly 30 years living in Canada.
She was married three times, but her last husband died in the 1970s and she moved back home in 1990.
Whilst she didn't qualify for a pension from 60, because she had been abroad and hadn't made national insurance contributions over here, she became eligible 20 years later.
Helen continued: "I expected if something like that was available, we would have been notified, but it was never suggested.
"I'm glad mother has it now, but it shouldn't have taken her getting to 100 to find out about it.
"£75,000 is a lot to have missed out on and I'm sure we aren't the only ones that didn't know.
"I have no idea why it has been kept so quiet but I encourage people to look into it and find out what they might be entitled to.
"I'm so grateful to Steve Webb for helping us."
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