Socci - if you've told them about all changes and they've still got it wrong there's a form they can give you to fill in to stop them reclaiming the money, but you usually have to ask, they don't publicise it well.
As to the formula for tax credits, I've managed to glean a tiny bit of info but don't shoot me if its wrong, the IR don't make it easy to confirm!!
CHILD TAX CREDIT - will be given to some amount as long as your (combined) income is under £58k. The threshold for CTC is £13,910. Under that you get the max CTC, over that you get 37p deducted for every £1 over you earn. But you're guaranteed a minimum of £545 a year if you're joint income is under £50k. As far as I can see the max is £1670.08 a year. However, that figure is worked out from IR site itself and my tax credit award for this year has awarded me more than that so its basically a joke.
WORKING TAX CREDIT - Threshold drops to £5,220 if you're eligible for this too, and the same applies - every £1 over gets 37p deducted from max. You get extra if you work over 30 hours a week or if someone in your family is disabled but how they work that out is beyond me. The max seems to be £2888.64 a year.
But a better way to work it out is times your dog's age by 300, add the number of times you've been to the loo in the last week and deduct the ages of your kids from that number - it'll probably come out more accurate.