Heisenberg - they are reducing the income threshold from 6,420 to 3,850, and increasing the taper rate from 41% to 48%.
An example of what this means:
Household income = £15k, 2 children, 2 adults (1 working over 30hrs, 1 not working), no child care costs
Current maximum tax credit amount = £8,875
Current income threshold = £6,420, so income over this is £8,580
The taper rate is applied to this figure - 41% of 8,580 is 3,518
This is taken from the maximum amount - 8875-3518 = £5,357 tax credit entitlement.
From April 2016:
Maximum tax credit amount = £8,875
Income threshold = £3,850, so income over this is £11,150
The taper rate is applied to this figure - 48% of 11,150 is 5,352
This is taken from the maximum amount - 8875-5352 = £3,523 tax credit entitlement.
That's a £1,834 pa reduction.