Certain changes in circumstances are notifiable to the tax credit office. If notifiable, you get one month to notify. If the change in circumstances is such that it reduces or eliminates your entitlement before means-testing abatement, then you have an obligation to notify within that month. If the change is such as to increase your entitlement before means-testing abatement then notification is optional but can only be backdated by a month so the deadline is effectively the same.
A mere increase in income arising from overtime work is not of itself a change in circumstances requiring notification within a month. As discussed earlier in this thread that increase in income would be notified as part of your annual declaration by 31 July 2013.
However the increase in working hours that gives rise to that increased income MIGHT be a change in circumstances which has the potential for increasing your headline award prior to means-testing abatement. This would fall within the optional notifications which can be back dated by at most a month and might suggest early notification. A typical example would be if you were doing fewer than 30 hours per week on average before overtime, but now increased to 30 hours per week or more.
It is also possible that your increased income in 2013-14 by reason of overtime or other factors is so great that, despite the income disregard, your provisional award for 2012-13 is overstated. More likely, your initial payments on account of 2013-14 when we get to next April would be overstated if the overtime continues. This could give rise to an overpayment of tax credits which the tax credit office would be entitled eventually to recover. If you are concerned at that possibility then you can protect yourself against overpayments by providing the tax credit office with an early estimate of income, well before you have a statutory obligation to notify.
The best advice in my opinion, if you are not sure of the rules and their effect, is to notify the tax credit office of your new hours and expected income immediately, whether you have an obligation or not. Keep careful notes of what you say on the telephone and the date and time. All calls are recorded and you may need to rely on this information if they act incorrectly (or fail to act at all) on the information that you provide.