Unfortunate SDP situation

We’ve just started advising this family. Is there anything we can do in this situation?
Mum and 4 children on Income support / tax credits. Eldest turned 5 and triggered family to move on to UC on 1st December 2019.
However Mum is on PIP (daily living) so should have been entitled to SDP on IS and remain on legacy. But Income Support had two older non dep sons living at the property. The sons in fact moved out in 2018.

UC is now in payment, is there any way back or could there be entitlement to transitional SDP? ‘

The managed migration regs tell us that the transitional payment is due where a person:

1.  No claim may be made for universal credit on or after 16th January 2019 by a single claimant who, or joint claimants either of whom—

(a)is, or has been within the past month, entitled to an award of an existing benefit that includes a severe disability premium;

So on that basis I’d say there might be entitlement. A lot weill depend on whether JCP have found out that the son’s moved out.

Also there’s the FOI request at this thread - https://www.rightsnet.org.uk/forums/viewthread/13872/P165/#73665 - which gives further advice -

Our aim is to identify any claimants who breach the SDP gateway as quickly as possible and before universal credit is awarded and paid.  Where claimants have not been prevented from claiming universal credit by the SDP gateway when they should have been, the Secretary of State for Work and Pensions can revise the decision to award universal credit and reinstate any DWP legacy benefits that have been terminated, so as to put claimants back in the position they were in, as regards DWP legacy benefits, before they claimed universal credit. 

Where it is in the claimant’s best interests to do so, the Secretary of State for Work and Pensions can revise the universal credit award decision.  The Secretary of State for Work and Pensions will consider cases that have breached the SDP gateway on a case by case basis.

Secretary of State for Work and Pensions will return claimants to DWP legacy benefits where:

- the error is identified promptly, prior to any award of universal credit, or
- returning the claimant is in their best interests (which may include financial interests).