Do you not have to apply for the tax credits first before you get them? I've just been trying to get as much info without calling the tax office as am in the office at moment and shouldn't really be looking into this, but once I start I like to know the answers.
Found this on HMRC about filling in your application form which might answer your question about overtime and bonus'
You might not have all the information you need to work out your income for the last tax year. If so, use what you do have to make an estimate.
For example, if the money you have coming in is roughly the same throughout the year, you can use that amount and multiply it by:
52 if you are paid weekly
12 if you are paid monthly
13 if you are paid every four weeks.
If your estimate is too low, the Tax Credit Office may pay you too much and you'll have to pay it back.
If you have estimated your income, put an 'x' in the estimated income box of your tax credits claim form or Annual Declaration form. You'll need to provide details of your actual income by 31 January or the deadline given to you when you're asked to renew your tax credits.