GOV.UK
Particular aspects: estimated income - testing - not investigating
When you consider the amount of the estimated figure, you are not expected to carry out an in-depth investigation of the underlying figures. If the customer is self-employed, for example, you should not ask to see the full set of business records, or attempt to construct a means test. You will simply be conducting a “reality check” to determine whether the figure is in approximately the right area, and forms a reasonable basis on which to make provisional payments of tax credit.
Remember that an estimate, by its very nature, cannot be an exact figure. This will be particularly true if the customer submits the estimate relatively early in the year, as the further away from the end of the year, the more scope there is for changes that will affect the amount.