Regarding the OP, what you're essentially saying is that you've happily received (albeit in error) and spent £1500 too much from the Gov't, and now you're a bit pissed that they've got the cheek to ask for it back?
Yes but who made the error?
Precisely! When you are notified of the award for the year the letter includes no hint of how it is calculated and the call centre staff don't know either. You have to trust that the amount they say you are entitled to is correct. If it isn't, it is usually only spotted in following years or by random audit.
Does it pay to work?
Between an annual wage of £10,000 and £20,000, the marginal rate of "taxation" (tax, NI, loss of benefits) is 70%. For every pound you earn, you only take home 30p. Factor in additional costs such as travel, child care (largely subsidised but still costly) work wear etc, quite often it costs money to go to work. It's easy to see why some people can't be bothered.
Give a man a gun, and he can rob a bank.
Give a man a bank, and he can rob everybody.