Ok, I think there is a lot of confusion on this thread.
I will answer renderedspeechless, and hopefully everyone will be clearer.
Tax credits is operated on a strict criteria, with a strict formula.
If two families have exactly the same composition (make up), circumstances (hours worked) and pre-tax income, they will get exactly the same Tax Credit award.
The first step is totalling all the 'elements' that apply to a family unit. This can be the confusing bit, because you have to see what applies.
1.Basic element £1,920
2.Couple and lone parent element £1,890
3.30 hour element £790
4.Disabled worker element £2,570
5.Severe disability element £1,095
6.50+ Return to work payment (16-29 hours)
7.Child Tax Credit Family element £545
8.Family element, baby addition £545
9.Child element £2,300
10.Disabled child element £2,715
11.Severely disabled child element £1,095
So, everyone with children gets
Element 1
Element 2
Element 7
and at least 1 x element 9.
But, if you work over 30 hours, you get element 3 added on.
If you have more than one child, you get the child element for each child.
If you have a baby (under 1) you get element 8 added on, etc.
So, let's take an example, of a single parent on £24k with one school aged child.
Element 1 (1920), because everyone gets it.
Element 2 (1890), because she is a lone parent
Element 3 (790) because she works 30 hrs +
Element 7 (545) because she has a child, so is a 'family'
Element 9 (2300) because she has one child.
Total (1920+1890+790+545+2300)= 7445
However, her earnings (pre-tax) will be taken into account. No deductions are made for the first £6420, so that can be ignored.
£24000-£6420 = 17580
Of the remaining 17580, the Tax Credits reduce by 39p for every £1 of earnings (pre-tax).
So, 17580 x 39/100 = £6856.20
Because her earnings are £17580 above the threshold, she loses £6856.20 of her Tax credits award.
7445-6856.20=588.80 for the year.
Now, I said it was very simple, and what I mean by that is that you can play with the figures for that scenario easily.
Movingbeds used the same circumstances but £18k pa earnings.
You can ignore the earlier parts of working out, because nothing has changed, so then you just calculate
18000 - 6420 = 11580
11580 x39/100 = 4516.20
Her award would be 7445-4516.2 = 2928.8 for the year.
So, dropping £6k in pre-tax income (£3660 in real money) would give an increase of £2340 in Tax credits.
In other words, you would be £1320 worse off by taking a drop of £6000 in salary.
That isn't taking into account housing benefit changes or any other benefit.