I have recently applied for working tax credits, the form I filled only asked for 2009/10 income, thus not requiring any info about last years income, that did surprise me also,I was on ESA long term illness rate.
The calculator isn't really too helpful, best you ask for tax credit form if you are going to work over 16 hrs, I have just started work for 20 hrs, I hope the tax credits will make up for the benefits I have lost.