I'm not actually sure about this - I'd speak to the CMS to ask them exactly what the rule is, especially since they'll be the ones who interpret it.
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If you don't get a reply from that, I would speak to your MP and get him to make a complaint - although they haven't done anything wrong as such, I would day that they should be looking for an answer so you know where you stand. As you say, it's simply unfair not to be able to have certainty one way or another.
You definitely need a decision in writing from them, so that they can't re-interpret the rule later on.
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You could try your MP again and specifically ask that he writes to the Secretary of State in charge of the DWP and see if that gets anywhere.
I have loads of issue with CMA, However my understanding is they will take the first year assessment base on the P60of the previous Tax year and if your salary is not effected by more than 25% the calculation of the maintenance we remain the same, until the following years review.