No Housing Cost Element on UC since 2019!

Looking for some advice as I racking my head to see if there is anyway this client might be able to get a very large amount of backdated Housing Costs from UC.

Situation was the client was in Temporary Accommodation when they made a Universal Credit claim in 2018, and at that point declared Housing Costs. They were advised on the Journal to claim HB due to being in Temp Accom.

They then moved into a Local Authority accommodation at the end of 2018, but didn’t do a change of circs on UC about where they lived until 2021. At this point they declared they didn’t have any Housing Costs.

She has only just a few months ago finally declared that she has housing costs, and declared this change from the date she moved in. She put a note on her journal asking if there is any scope for a late revision and to be backdated the housing costs, declaring vulnerabilities.

Other points to mention;
-Landlord had requested deductions from UC for rent arrears payments
-In 2021 Landlord also contacted UC asking for an APA for the Housing Costs, however obviously as there were no HCE in payment this wasn’t agreed. UC did put a note on the journal asking the client to declare Housing Costs, but she still didn’t.

UC have replied to her message on the journal explaining that as the request is to look at a decision over 13 months ago, they are unable to make a MR decision and she has no right of appeal.

Looking into it, it feels like it would be possible to do an anytime revision if she had been in the Local Authority Property when she made the UC claim, but as she was already in receipt of UC when the Change of Circumstance of moving into the LA property occurred then it is restricted to the 13 month rules.

Am I missing anything? Any chance of even getting some of the HCE backdated? This is potentially £20,000 of HCE that she has missed out on, and has just been holding off from her arrears building up too much by paying out of her standard allowance and borrowing from Friends and Family.