How on earth do those people expect you to maintain payments at the contractual level when you have lost £50 of income! Which firm are you with and have you considered trying to speak to your IP directly?
Why has yor tax cedit reduced so much, did you earn more last year than was estimated, also do check that they are using correct estimated income for this current tax year, you will find this on your award notices
It seems a severe drop and check it is not to repay an overpayment. If this is the case the overpayment may be captured by the IVA but this would depend on a number of factors. As Melanie says you should speak to your IP directly.