You should probably opt back into the Child Benefit system. You can do this by using an online form at the HMRC website, or by calling the Child Benefit Helpline. If your taxable income is less than £50,000/year then you can spend all of the benefit. If it is between £50,000 and £60,000/year, you should keep some aside for repaying HMRC once your tax return has been submitted.
Ways to reduce your taxable income (and potentially qualify for more child benefit) include paying into a personal pension scheme; and transferring deposits and investments to a spouse. Also, if you have your own business, you could discuss with your accountant possible ways to reduce your income or move it into a lower earning tax year.