I am under the impression that they cannot insist you pay it all at once, and, for instance, if you received £20 per week, they cannot takemore than that back, so if you owe £500 which iwas overpaid at £20 per week then you would have at least 25 weeks to pay it back in. Personally I would show them your outgoings, and tell them what you CAN afford - even if it's £5 a week.
Catastrophica is right though, it makes a mockery of the whole thing - one month you might get an extra £50 which you have been relying on, but the next month not only do they not give you that, they demand you pay THEM!!
MP is a good idea, and speaking to a higher officer who has some dicretion to agree a repayment schedule which suits you.
Shel