I have had quite a few cases where DWP debt management issue a “decision” that IS has been overpaid, but no entiitlement decisions are notified. The IS overpayment decision is clearly invalid in these circumtances.
This is where you will fall foul of CH/872/2009, but in any case the end of IS of itself is not sufficient for a superseding decision ending HB/CTB (CH/3736/2006, CH/1664/2009)
What is more, the mere determination that the person has acquired a partner is not of itself sufficient for a superseding decision ending IS (CIS/4434/2004).
If the DWP have not done a full re-assessment and notified the new entitlement decisions to the claimant, any competent representative could make it very difficult for the LA to get its case off the ground