ESA pending appeal against IB conversion refusal

My client is receiving assessment-phase rate ESA pending an appeal against a decision that he cannot transfer to ESA from IB. Since the ESA was awarded, he has been awarded the middle rate of the care component of DLA and now meets all the criteria for entitlement to a Severe Disability Premium.
  Because we understand his ESA award to be cESA only, we rang the new claims helpline to check if he would need to make a new claim for irESA in order to avail himself of the premium. We were told he wouldn’t and that he should simply notify the ESA office of the DLA award and request a reassessment.
  We did that and pointed out that we were doing so with a view to the client being awarded irESA with an SDP, but that hasn’t happened. The letter we sent making the request has simply been forwarded to HMCTS and a copy has been returned to me by them as ‘further evidence’ for the appeal.
    Now, I’m fairly sure that a new claim is not required for an existing cESA-only claimant to access irESA, but is there anything which prevents premiums being paid to claimants during the period that they receive ESA pending an appeal? It was very clear from the wording of my letter that we were after a change of circumstances re-assessment of the award pending the appeal and that we were not citing the DLA award as evidence that the original conversion decision was incorrect. That was done in a separate communication.

A letter reporting the change of circumstances has always worked for me, but see
http://www.rightsnet.org.uk/forums/viewthread/1038/