HMRC says:
Eligible tax credits only customers set to receive first Cost of Living Payment
Most customers receiving both tax credits and other eligible benefits will have received their first Cost of Living Payment from the Department for Work and Pensions in July.
Eligible customers on tax credits only will receive their first payment from us between 2 to 7 September. All payments go automatically into the bank account where they receive their tax credits payments.
If customers believe they’re eligible but haven’t received a payment by these dates, they should wait until 16 September at the earliest to contact us, to allow time for their payment to be processed by their bank, building society or credit union.
We won’t be able to provide customers with any further information before this date.
The second payment will be made in the winter.
We have also developed a toolkit to help you support your networks with the cost of living.