That seems right, CherryMonster - you should get around £330 per week child tax credit but you won't qualify for working tax credit.
The reason is that while Child Tax Credit is variable depending on your family makeup, the rate for Working Tax Credit is fixed:
Basic element £1,920
Couple and lone parent element £1,970
30 hour element £790
Disabled worker element £2,855
Severe disability element £1,220
So unless you have a disability, the total Working Tax Credit award for a family with at least one parent working full-time is:
£1920+£1970+£790 = £4680
You're allowed to earn £6420 without penalty, then for every £1 you earn after that, 41p is deducted.
£4680/41 x 100 = £11414.63
Add on the amount you're allowed to earn without penalty:
£11414 + £6420 = £17834
If you have a joint income of £17835 or more, you won't get Working Tax Credit no matter how many children you may have.