backdating of premiums after ESA ends

Cl. was on ESA and applied for PIPS. This was only given after an appeal which took fifteen months, award made from March 2017.

In the meantime cl. had gone onto Pension Credit, from May 18.

Cl’s partner would be eligible for underlying CA for part of that time (stopped work in October 2017) and Carer premium could be added to ESA award. The only problem is that the ESA award is now closed and has been since May this year. Could the ESA award be reivisited to allow the addition of the Carer Premium ( and payment of arrears)or is it now too late?

I can’t find anything in CPAG to say one way or the other and I was wondering if anyone had tried this and what happened?