Working tax credit run-on – If you stop work or your normal working hours fall below the minimum required for a working tax credit (WTC) award, which is either 16 or 30 hours a week (or 24 hours a week for some couples with children), you will be treated as being in work for a further four weeks, allowing for a 4-week run-on of WTC. You must report your change of circumstances to HMRC within one month to receive the run-on.
WTC(E&MR) Regs, reg 7D

Self-employment – If you are claiming WTC to top up earnings from self-employment, the employment needs to aim to be profitable and must be commercial, organised and regular. If your income from self-employment falls below the hours you declare that you work, multiplied by the national living wage or national minimum wage that applies in your case (see Box I.5 ), then you may be asked for evidence that you meet this test.