Which tax years affect which payments for child benefit

Hi,

I'm having trouble figuring out which tax year's earnings can affect which year's child benefit payments.
Last tax year (April 2012 - April 2013) I earnt around 55k in total with overtime and on call payments. However this tax year I've upped pension contributions and am taking most overtime as time off so hope to keep at or below the 50k threshold.

As the changes came in January this year, would last tax year's earnings affect the benefit from January to this April, and this year's earnings affect this year's payments? I.e. are likely to only have to pay back some of the benefit received from Jan to April this year as long as I stay under 50k this tax year?