In 2011, if you have children and your income is over £41,300, you can not claim tax credits.

If you are single and do not have children, the limit of your income to claim tax credits is £12,900.

If you are married and do not have children, the limit of your income to claim tax credits increase to £17,700. So you will be better off to get married if you don't have children.

For latest information, you can check HM Revenue & Customs site.