I think the first place to look is D&A Reg 7, so you can establish the precise ground on which you are making a superseding decision. Reg 7 contains various special rules for RO valuations after 52 weeks and for redeterminations and substitute determinations, but so far as I can see no special rule exists for an RO determination made after a rent increase. Therefore, the superseding decision must be made under the general change of circumstance provision in Reg 7(2)(a).
The effective date is set by Regs 8(2) & (3): that is, the change takes effect in the normal way under HB Reg 79 (from the day of the increase) [b:ea268df637]but[/b:ea268df637] for an advantageous change reported outside the one-month time limit the day of the increase must be viewed through the prism of Reg 8(3) which requires you to pretend that the rent increased on the day you were told about it. Therefore the correct effective date for the superseding decision is the exact day on which you were told of the rent increase.
If for some reason the new RO valuation is lower, then it’s a disadvantageous change from the real life date of the rent increase and the claimant will have been overpaid since then. That’ll teach him.