If there is a written agreement giving the claimant exclusive possession of the whole property for the full rent, I think that is a hefty factor counting against the inference that there is a separate de facto tenancy between the landlord and the putative non-dep.
If you do decide to go along with the view that there is actually a separate oral tenancy between the non-dep and the landlord, and that the claimant’s true rent liability is therefore reduced accordingly, Reg 3 would not expressly rule out the applkication of a non-dep deduction when the person reaches age 25 or comes off JSA, but it is surely absurd to double-count: otherwise, in any house-share arrangement where the occupiers have indivual tenancy agreements but there is a shared living room or kitchen you could argue that everyone is everyone else’s non-dep! The only sensible interpretation is to say that the categories of non-dep and fellow HMO tenant are mutually exclusive.
