I dont think Council Tax case law has any relevance, and you should refer to the established social security case law on living together as husband and wife.
At the end of the day, the question is not a matter of law but of fact. A decision maker or Tribunal must come to a decision on the evidence. Exercising the RTB jointly is on the face of it evidence that suggests they are still a couple, but its not conclusive because it may be the only way they can get a mortgage.
I know of no recent decisions that seriously develop the law from the days of supplementary benefit
