The whole tax credit system was ludicrous when Gordon Brown introduced it. I was very happy on a very good salary (for the time) yet, as a family, we qualified to receive tax credits. It was deliberately poorly (widely) targeted to buy votes from middle income earners.
And the tax credit to pay for childcare was ludicrous by design
It was base don your earnings 1-2 yrs previous when obviously a fair number of claimants would have had someone on maternity/paternity leave due to the reason for the credit. They then paid you and then said "oh we overpaid",
we actually updated throughout the year on our changing earnings but the main ICL computer had a glitch and all accounts open when it broke were locked - that led to nearly 2 years of a biweekly physical cheque being sent to us, they also sent every letter in duplicate to both my wife and myself.
At one point within the last 2 week period as they stopped the manual cheques it went like this
!. You owe back £1500 in overpaid benefit, us "really how we told you all changes as they happened, please explain". You need a calculation notice we will send one
2. 4 letters arrive 2 for each of us, 2 saying "thank you for calling us a calculation notice will follow and 2 in same post which just said "you told us x the answer is £1200" with no explanation
3. We rang and said "how come same figures from us have now given £1200 not £1500 and we dont think even that, they said we will look back into it.
4. 2 random letter arrive saying they owe us £350 and another letter with a cheque for that
5 Another set of 2 letters saying they have looked into our appeal and they think actually they us £15
6. We rang said "please close the query, we will take the last £15 calculation as correct, we won't bank the £350 just leave it and we will move to busybees nursery vouchers instead.
7. A letter arrived with £15 cheque
6 months later they wrote to say the £350 was an error and they wanted it back (we had never cashed it) and a week later wrote to remind us to cash the £15 cheque (we didnt). Both of those letters were send in duplicate. We never heard from them again