Final tax credit award 23/24 increased now eligible for backdated col 23/24 payments

I’m being passed around the houses between HMRC and DWP and not getting anywhere. I updated mine and my partners income at the end of year review for 23/24 which then generated an award for child tax credits which was paid in a lump sum earlier this month. At the time the COL payments were made I didn’t qualify but now the end of year final award generated a payment, according to the gov website the cost of living x3 payments for 23/24 should be backdated. HMRC say that as I have now switched to UC because I had to transition, as received a letter from them, in May 2024 it’s DWP’s responsibility. DWP say I don’t qualify as not claimed UC since transitioning in May 24. My issue is this relates to 23/24 when it was tax credits not UC. But neither department can give me an answer. I’d like to know which department is responsible for sorting the col mop up payments for 23/24 out and to hopefully sort out a backdated payment of £900 which I believe I am entitled to as per the Gov website. Hope someone can help please

Hi,
If you received a payment of tax credits for any day in the periods outlined on gov.uk, HMRC would arrange for the mop up payments to be issued.
At present the mop up payment dates have not been published:
Cost of Living Payments 2023 to 2024 
Thank you.

Hi, Thank you for clarifying it’s HMRC which is what I thought. I received a lump sum payment in August 2024 for 23/24 tax credits award after finalising the end of year review. Therefore I qualify for the last 2 COL payments £300 and £299 as those qualifying dates are Nov/Dec and Aug/Sept 2023. Hopefully this will be paid soon via the mop up payments.