I don’t think they can do this. I assume they are working on the basis that once a decision is made then, unless there is a relevant change, it will be 52 weeks before they can make another decision.
But in this case the starting point has to be whether or not a referral is required. HB Reg 14(1)(a) insists that you “shall” refer the rent to the Rent Officer because it’s clear from what you say that the existing decision can not be applied to this claim.
Now if you “shall” refer the case (i.e. you must) then Reg 3(1) of the the Rent Officer (Housing Benefit) Functions Order says that they “shall” (i.e. must) make a determination. There are circumstances in which the Rent Officer can refuse but these relate only to specified circumstances that don’t apply to your case or where the application “is not one which gives rise to a duty to make a determination” (Reg 6(4) of the same regs).
So I don’t see how the RO has any choice but to give you a decision.