Thank you Twinklemegan, I think that specifics sometimes clarifies points, but leaves people open to direct attack!
Looking at this thread from a more objective angle, I think that there has been a general misconception about the purpose of tax credits, where in general people have seen them as credits against tax paid, rather than payments to supplement income.
For posters with generous incomes, it has come as a shock that for some families the tax credits exceed tax paid. But for the government who brought in this system, which is a re-distributive one, this was always the intention. The only alternative would be to raise the minimum wage to acceptable levels, but that would cost too much because everyone would have a decent wage. So instead they target people (mainly) with children and give added incentives for those that choose to work a significant number of hours.
Cleverly, the government have ensured that in general, if both parents are working, then they have to pay for the childcare costs themselves, because in most cases 2 incomes of minimum wage would take them over the income threshold for childcare, and it is capped beyond 1 child any way.
Perhaps the issue here is not so much one of the system, but the public's perception of that system, which has prooved inaccurate.