As stated in my link, tax credits are initially always based upon the PREVIOUS tax year's total earnings (but are then later adjusted to take into account any changes in your income).
Quote:
"The calculator asks you for income for the last tax year. But if your income for this tax year (6 April 2011 to 5 April 2012) is likely to be lower, type in an estimate of this year’s income instead.
It's important to know that when you make your actual claim, you must enter last year’s income on the form. Your first tax credits award is always based on your previous year’s income"