Some of the hundreds of thousands of people struggling to pay back tax credits which the government says were erroneous are likely to have been underpaid, experts have warned.
Cheryl McGrath: More than 30 tax statements and dozens of letters
The Low Incomes Tax Reform Group said that in cases where families had received multiple award notices it was almost impossible to keep track of their payment schedules.
Tax experts at the charity found that a single mother, Cheryl McGrath, 44, from Reading, did not owe the £2,200 that the Revenue had calculated - but in fact was underpaid by about £1,150.
"This could be the tip of a very big iceberg," Robin Williamson told Real Story.
"It makes you very suspicious that the Revenue are not producing the right results."
"It's well known that their systems weren't working as well as they should have been in 2003-4."
Ms McGrath, a special needs teaching assistant on a £7,300 salary with two sons, has received more than 30 award notices since applying for child tax credit and working tax credit under the new system from April 2003.
When Mr Williamson and his colleagues checked these against her bank statements, it showed that several substantial payments had not made it into her account.
It was possible that the Revenue's computer system had not linked up with her employer's, he said.
In April 2004, Ms McGrath was told she had been overpaid and that £260 a month would be clawed back from her current tax credits.
After appealing against this on grounds of hardship, her tax credit payments were resumed only to drop again in March this year.
By the summer, Cheryl had sunk deep into debt.
"The second week in August, I couldn't feed my children," she said.
"I'd borrowed as much money as I could from everyone I could think of. I had a friend who was actually helping me out with food."
In August 2005, the Revenue admitted that the overpayment was their mistake and agreed to return the money, in line with their code of practice.
But Ms McGrath has already put her house on the market to repay the extra debts she has incurred from bank charges, interest on credit cards and money owed to friends.
She has received £215 in compensation.
In another case dealt with by the Low Incomes Tax Reform Group, a mother had received several contradictory assessments on the same day.
One said she had been overpaid for 2002-3 which was impossible, given that the family tax credits system was not implemented until April 2003.
The other award notice stated that, for the purposes of her child tax credit, she had two children, but for the purposes of her child care element (which is attached to working tax credit), she had six children.
"It's illustrative of the kind of nonsense the computer produces," said Mr Williamson.
However, a spokesman for the Revenue, Patrick O'Brien, told Real Story that he did not believe large numbers of claimants had been underpaid.
"Customers may have received interim payments from their local office and thus these payments may not feature in your calculations," he said.
"We do get calculations right, first time, most of the time."
Other evidence suggests the system is not helping the low income families it was designed for.
One father's £50,000 salaried household was mistakenly paid £10,000 in child tax credits (family element). The overpayment was deducted in instalments from his continuing award.
"He was laughing," recalled Mr Williamson. "He'd effectively been given an interest-free £10,000 loan.
"For a low income family it would have been disastrous."
The Low Incomes Tax Reform Group has found that families on incomes of up to £80,000 can still receive some tax credit, if they have the right number of children and are paying enough in childcare charges to qualify for maximum childcare support.
Charities representing those on low incomes have urged the government to simplify the tax statements.
"It took three of us who regard ourselves as experts several hours each to go through Cheryl's award notices and link them with her bank statements," said Mr Williamson.
In the Real Story programme, a team of students from Oxford University's Corpus Christi College failed to decipher a tax credit statement using the same guidance notes given to the public.
One was working towards a doctorate in applied mathematics.
Mr Williamson said the statements should come with an explanation of how the award was calculated. But claimants should also be aware they could dispute an assessment.
"It doesn't occur to them to question a government department about their awards. If they have any doubts, they should seek independent advice."
Speaking to Real Story, a Revenue spokeswoman apologised for the mistakes made in assessing Cheryl McGrath's tax credits.
"We have ensured she has all the payments she's entitled to - just over £9,000 for each of the years.
"It's taken us too long to sort it out and we're really sorry for that."
Real Story: Tax credit chaos: BBC ONE on Monday 14 November at 1930 GMT.