Yup, you got your answer, well done. Tax Credits are excluded.
But when it gets round to "next year", you might want to have a go at doing it (or at least starting it) in the summer rather than waiting until 3 weeks before the filing deadline. Your stress levels will be much reduced. You will know your tax position earlier. The helpline will be less crowded if you need it, and you will have plenty of time to sort out any problems or queries.
But perhaps more to the point, being a tax credit claimant you will have a deadline of 31 July 2013 by which to report to the Tax Credit office your taxable income for 2012-13 tax year and renew your claim. Estimated income figures are at that stage permitted but it is so much smoother to provide actuals (as well as reducing the risk of overpayments). So if you get the tax return completed in time for the tax credit deadline you will have an easy ride.